diff --git a/src/02-common-principles.md b/src/02-common-principles.md
index 7888bd7fd9..85343b8ab6 100644
--- a/src/02-common-principles.md
+++ b/src/02-common-principles.md
@@ -149,6 +149,14 @@ data from subject `01`.
 The suffix `eeg` and the extension `.edf` depend on the imaging modality and
 the data format and indicate further details of the file's contents.
 
+In cases where entities duplicate metadata,
+the presence of an entity should not be used as a replacement for
+the corresponding metadata field.
+For instance, in echo-planar imaging MRI,
+the [`dir-<label>`](./99-appendices/09-entities.md#dir) entity MAY be used
+to distinguish files with different phase-encoding directions,
+but the file's `PhaseEncodingDirection` can only be specified as metadata.
+
 A summary of all entities in BIDS and the order in which they MUST be
 specified is available in the [entity table](./99-appendices/04-entity-table.md)
 in the appendix.
@@ -552,16 +560,16 @@ it refers to. For example `sub-01` refers to the `sub` entity (a
 subject) with the label `01`. The `sub-` prefix is not part of the subject
 label, but must be included in file names (similarly to other key names).
 
-## Uniform Resource Indicator 
+## Uniform Resource Indicator
 
 A Uniform Resource Indicator (URI) is a string referring to a resource and SHOULD
-have the form `<scheme>:[//<authority>]<path>[?<query>][#<fragment>]`, as specified 
-in [RFC 3986](https://tools.ietf.org/html/rfc3986). 
-This applies to URLs and other common URIs, including Digital Object Identifiers (DOIs), 
-which may be fully specified as `doi:<path>`, 
+have the form `<scheme>:[//<authority>]<path>[?<query>][#<fragment>]`, as specified
+in [RFC 3986](https://tools.ietf.org/html/rfc3986).
+This applies to URLs and other common URIs, including Digital Object Identifiers (DOIs),
+which may be fully specified as `doi:<path>`,
 for example, [doi:10.5281/zenodo.3686061](https://doi.org/10.5281/zenodo.3686061).
 A given resource may have multiple URIs.
-When selecting URIs to add to dataset metadata, it is important to consider 
+When selecting URIs to add to dataset metadata, it is important to consider
 specificity and persistence.
 
 Several fields are designated for DOIs, for example, `DatasetDOI` in `dataset_description.json`.
diff --git a/src/04-modality-specific-files/01-magnetic-resonance-imaging-data.md b/src/04-modality-specific-files/01-magnetic-resonance-imaging-data.md
index 7d74db3bea..f52e2b38ea 100644
--- a/src/04-modality-specific-files/01-magnetic-resonance-imaging-data.md
+++ b/src/04-modality-specific-files/01-magnetic-resonance-imaging-data.md
@@ -567,39 +567,37 @@ JSON example:
 
 ## Fieldmap data
 
-Data acquired to correct for B0 inhomogeneities can come in different forms. The
-current version of this standard considers four different scenarios. Please note
-that in all cases fieldmap data can be linked to a specific scan(s) it was
-acquired for by filling the IntendedFor field in the corresponding JSON file.
-For example:
+Data acquired to correct for *B<sub>0</sub>* inhomogeneities can come in different forms.
+The current version of this standard considers four different scenarios:
 
-```JSON
-{
-   "IntendedFor": "func/sub-01_task-motor_bold.nii.gz"
-}
-```
+ 1. [Phase-difference map](#case-1-phase-difference-map-and-at-least-one-magnitude-image)
+ 1. [Two phase maps](#case-2-two-phase-maps-and-two-magnitude-images)
+ 1. [Direct *field mapping*](#case-3-direct-field-mapping)
+ 1. ["*PEpolar*" fieldmaps](#case-4-multiple-phase-encoded-directions-pepolar)
 
-The IntendedFor field may contain one or more filenames with paths relative to
-the subject subfolder. The path needs to use forward slashes instead of backward
-slashes. Here's an example with multiple target scans:
+These four different types of field mapping strategies can be encoded
+using the following image types:
 
-```JSON
-{
-   "IntendedFor": ["ses-pre/func/sub-01_ses-pre_task-motor_run-1_bold.nii.gz",
-                   "ses-post/func/sub-01_ses-post_task-motor_run-1_bold.nii.gz"]
-}
-```
+| **Name**         | `suffix`         | **Description**                                                                                                                                                                                                      |
+| ---------------- | ---------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| Magnitude        | magnitude\[1,2\] | Field-mapping MR schemes such as gradient-recalled echo (GRE) generate a Magnitude image to be used for anatomical reference. Requires the existence of Phase, Phase-difference or Fieldmap maps.                    |
+| Phase            | phase{1,2}       | Phase map generated by GRE or similar schemes, each associated with the first (`phase1`) or second (`phase2`) echoes in the sequence.                                                                                |
+| Phase-difference | phasediff        | Some scanners subtract the `phase1` from the `phase2` map and generate a unique `phasediff` file. For instance, this is a common output for the built-in fieldmap sequence of Siemens scanners.                      |
+| Fieldmap         | fieldmap         | Some MR schemes such as spiral-echo (SE) sequences are able to directly provide maps of the *B<sub>0</sub>* field inhomogeneity.                                                                                     |
+| EPI              | epi              | The phase-encoding polarity (PEpolar) technique combines two or more Spin Echo EPI scans with different phase encoding directions to estimate the underlying inhomogeneity/deformation map.                          |
+
+Two OPTIONAL entities, following more general rules of the specification,
+are allowed across all the four scenarios:
+
+  - The OPTIONAL [`run-<index>`](../99-appendices/09-entities.md#run) key/value pair corresponds to a one-based index
+    to distinguish multiple fieldmaps with the same parameters.
 
-The IntendedFor field is OPTIONAL and in case the fieldmaps do not correspond to
-any particular scans it does not have to be filled.
+  - The OPTIONAL [`acq-<label>`](../99-appendices/09-entities.md#acq) key/value pair corresponds to a custom label
+    the user may use to distinguish different set of parameters.
 
-Multiple fieldmaps can be stored.
-In such case the [`run-<index>`](../99-appendices/09-entities.md#run) key/value pair should be
-used. The OPTIONAL [`acq-<label>`](../99-appendices/09-entities.md#acq)
-key/value pair corresponds to a custom label
-the user may use to distinguish different set of parameters.
+### Types of fieldmaps
 
-### Case 1: Phase difference image and at least one magnitude image
+#### Case 1: Phase-difference map and at least one magnitude image
 
 Template:
 
@@ -609,32 +607,38 @@ sub-<label>/[ses-<label>/]
         sub-<label>[_ses-<label>][_acq-<label>][_run-<index>]_phasediff.nii[.gz]
         sub-<label>[_ses-<label>][_acq-<label>][_run-<index>]_phasediff.json
         sub-<label>[_ses-<label>][_acq-<label>][_run-<index>]_magnitude1.nii[.gz]
+        sub-<label>[_ses-<label>][_acq-<label>][_run-<index>]_magnitude2.nii[.gz]  # OPTIONAL
 ```
 
-OPTIONAL
+where
+the REQUIRED `_phasediff` image corresponds to the phase-drift map between echo times,
+the REQUIRED `_magnitude1` image corresponds to the shorter echo time, and
+the OPTIONAL `_magnitude2` image to the longer echo time.
 
-```Text
-sub-<label>/[ses-<label>/]
-    fmap/
-        sub-<label>[_ses-<label>][_acq-<label>][_run-<index>]_magnitude2.nii[.gz]
-```
+Required fields:
 
-This is a common output for build in fieldmap sequence on Siemens scanners. In
-this particular case the sidecar JSON file has to define the Echo Times of the
-two phase images used to create the difference image. `EchoTime1` corresponds to
-the shorter echo time and `EchoTime2` to the longer echo time. Similarly
-`_magnitude1` image corresponds to the shorter echo time and the OPTIONAL
-`_magnitude2` image to the longer echo time. For example:
+| **Key name**   | **Requirement level** | **Data type**            | **Description**                                             |
+| -------------- | --------------------- | ------------------------ | ----------------------------------------------------------- |
+| EchoTime1      | REQUIRED              | [number][]               | The time (in seconds) when the first (shorter) echo occurs. |
+| EchoTime2      | REQUIRED              | [number][]               | The time (in seconds) when the second (longer) echo occurs. |
+
+In this particular case, the sidecar JSON file
+`sub-<label>[_ses-<label>][_acq-<label>][_run-<index>]_phasediff.json`
+MUST define the time of two echos used to map the phase and finally calculate
+the phase-difference map.
+For example:
 
 ```JSON
 {
    "EchoTime1": 0.00600,
-   "EchoTime2": 0.00746,
-   "IntendedFor": "func/sub-01_task-motor_bold.nii.gz"
+   "EchoTime2": 0.00746
 }
 ```
 
-### Case 2: Two phase images and two magnitude images
+#### Case 2: Two phase maps and two magnitude images
+Similar to case 1, but instead of a precomputed phase-difference map, two
+separate phase images and two magnitude images corresponding to first and
+second echos are available.
 
 Template:
 
@@ -649,18 +653,24 @@ sub-<label>/[ses-<label>/]
         sub-<label>[_ses-<label>][_acq-<label>][_run-<index>]_magnitude2.nii[.gz]
 ```
 
-Similar to the case above, but instead of a precomputed phase difference map two
-separate phase images are presented. The two sidecar JSON files need to specify
-corresponding `EchoTime` values. For example:
+Required fields:
+
+| **Key name**   | **Requirement level** | **Data type**            | **Description**                                                                   |
+| -------------- | --------------------- | ------------------------ | --------------------------------------------------------------------------------- |
+| EchoTime       | REQUIRED              | [number][]               | The time (in seconds) when the echo corresponding to this phase map was acquired. |
+
+Each phase map has a corresponding sidecar JSON file to specify its corresponding `EchoTime`.
+For example, `sub-<label>[_ses-<label>][_acq-<label>][_run-<index>]_phase2.json` may read:
 
 ```JSON
 {
-   "EchoTime": 0.00746,
-   "IntendedFor": "func/sub-01_task-motor_bold.nii.gz"
+   "EchoTime": 0.00746
 }
 ```
 
-### Case 3: A real fieldmap image
+#### Case 3: Direct *field mapping*
+In some cases (for example GE), the scanner software will directly reconstruct a
+*B<sub>0</sub>* field map along with a magnitude image used for anatomical reference.
 
 Template:
 
@@ -672,12 +682,12 @@ sub-<label>/[ses-<label>/]
        sub-<label>[_ses-<label>][_acq-<label>][_run-<index>]_fieldmap.json
 ```
 
-In some cases (for example GE) the scanner software will output a precomputed
-fieldmap denoting the B0 inhomogeneities along with a magnitude image used for
-coregistration.
-In this case the sidecar JSON file needs to include the units of the fieldmap.
-The possible options are: Hertz (`Hz`), Radians per second (`rad/s`), or Tesla
-(`T`).
+Required fields:
+
+| **Key name**   | **Requirement level** | **Data type**            | **Description**                                                                    |
+| -------------- | --------------------- | ------------------------ | ---------------------------------------------------------------------------------- |
+| Units          | REQUIRED              | [string][]               | Units of the fieldmap: Hertz (`Hz`), Radians per second (`rad/s`), or Tesla (`T`). |
+
 For example:
 
 ```JSON
@@ -687,7 +697,17 @@ For example:
 }
 ```
 
-### Case 4: Multiple phase encoded directions ("pepolar")
+See [Using `IntendedFor` metadata](#using-intendedfor-metadata)
+for details on the `IntendedFor` field.
+
+#### Case 4: Multiple phase encoded directions ("pepolar")
+The phase-encoding polarity (PEpolar) technique combines two or more Spin Echo
+EPI scans with different phase encoding directions to estimate the distortion
+map corresponding to the nonuniformities of the *B<sub>0</sub>* field.
+These `_epi.nii[.gz]` files can be 3D or 4D --
+in the latter case, all timepoints share the same scanning parameters.
+Examples of software tools using these kinds of images are FSL TOPUP,
+AFNI `3dqwarp`, and SPM.
 
 Template:
 
@@ -698,17 +718,20 @@ sub-<label>/[ses-<label>/]
         sub-<label>[_ses-<label>][_acq-<label>][_ce-<label>]_dir-<label>[_run-<index>]_epi.json
 ```
 
-The phase-encoding polarity (PEpolar) technique combines two or more Spin Echo
-EPI scans with different phase encoding directions to estimate the underlying
-inhomogeneity/deformation map. Examples of tools using this kind of images are
-FSL TOPUP, AFNI 3dqwarp and SPM. In such a case, the phase encoding direction is
-specified in the corresponding JSON file as one of: `i`, `j`, `k`, `i-`, `j-`,
-`k-`. For these differentially phase encoded sequences, one also needs to
-specify the Total Readout Time defined as the time (in seconds) from the center
-of the first echo to the center of the last echo (aka "FSL definition" - see
-[here](https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/topup/Faq#How_do_I_know_what_phase-encode_vectors_to_put_into_my_--datain_text_file.3F) and
-[here](https://lcni.uoregon.edu/kb-articles/kb-0003) how to calculate it). For
-example
+The [`dir-<label>`](../99-appendices/09-entities.md#dir) entity is REQUIRED
+for these files.
+This key-value pair MUST be used in addition to
+the REQUIRED `PhaseEncodingDirection` metadata field
+(see [File name structure](../02-common-principles.md#file-name-structure)).
+
+Required fields:
+
+| **Key name**           | **Requirement level** | **Data type** | **Description**                                                                    |
+| ---------------------- | --------------------- | ------------- | ---------------------------------------------------------------------------------- |
+| PhaseEncodingDirection | REQUIRED              | [string][]    | See [in-plane spatial encoding](#in-plane-spatial-encoding) table of fields.       |
+| TotalReadoutTime       | REQUIRED              | [number][]    | See [in-plane spatial encoding](#in-plane-spatial-encoding) table of fields.       |
+
+For example:
 
 ```JSON
 {
@@ -718,15 +741,41 @@ example
 }
 ```
 
-The `label` value of the [`dir-<label>`](../99-appendices/09-entities.md#dir) key/value pair
-can be set to arbitrary alphanumeric label (`[a-zA-Z0-9]+` for
-example `LR` or `AP`) that can help users to distinguish between different
-files, but should not be used to infer any scanning parameters (such as phase
-encoding directions) of the corresponding sequence. Please rely only on the JSON
-file to obtain scanning parameters. \_epi files can be a 3D or 4D - in the
-latter case all timepoints share the same scanning parameters. To indicate which
-run is intended to be used with which functional or diffusion scan the
-IntendedFor field in the JSON file should be used.
+See [Using `IntendedFor` metadata](#using-intendedfor-metadata)
+for details on the `IntendedFor` field.
+
+As for other EPI sequences, these field mapping sequences may have any of the
+[in-plane spatial encoding](#in-plane-spatial-encoding) metadata keys.
+However, please note that `PhaseEncodingDirection` and `TotalReadoutTime` keys
+are REQUIRED for these field mapping sequences.
+
+### Expressing the MR protocol intent for fieldmaps
+
+Fieldmaps are typically acquired with the purpose of correcting one or more EPI
+scans under `func/` or `dwi/` for distortions derived from *B<sub>0</sub>*
+nonuniformity.
+This linking between fieldmaps and their targetted data MAY be encoded with the
+`IntendedFor` metadata.
+
+#### Using `IntendedFor` metadata
+
+Fieldmap data MAY be linked to the specific scan(s) it was acquired for by
+filling the `IntendedFor` field in the corresponding JSON file.
+
+| **Key name** | **Requirement level** | **Data type**                         | **Description**                                                                                                                                                                                                                                                                 |
+| ------------ | --------------------- | ------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| IntendedFor  | RECOMMENDED           | [string][] or [array][] of [string][] | Contains one or more filenames with paths relative to the participant subfolder. The path needs to use forward slashes instead of backward slashes. This field is OPTIONAL, and in case the fieldmaps do not correspond to any particular scans, it does not have to be filled. |
+
+For example:
+
+```JSON
+{
+   "IntendedFor": [
+        "ses-pre/func/sub-01_ses-pre_task-motor_run-1_bold.nii.gz",
+        "ses-pre/func/sub-01_ses-pre_task-motor_run-2_bold.nii.gz"
+    ]
+}
+```
 
 <!-- Link Definitions -->
 
diff --git a/src/99-appendices/09-entities.md b/src/99-appendices/09-entities.md
index 32acf9fbe3..a605bec91e 100644
--- a/src/99-appendices/09-entities.md
+++ b/src/99-appendices/09-entities.md
@@ -103,8 +103,8 @@ Full name: Phase-Encoding Direction
 
 Format: `dir-<label>`
 
-Definition: The `dir-<label>` key/value can be used to distinguish
-different phase-encoding directions.
+Definition: The `dir-<label>` key/value can be set to an arbitrary alphanumeric label
+(for example, `dir-LR` or `dir-AP`) to distinguish different phase-encoding directions.
 
 ## run
 
diff --git a/src/schema/entities.yaml b/src/schema/entities.yaml
index ae609b49e3..eecff7353c 100644
--- a/src/schema/entities.yaml
+++ b/src/schema/entities.yaml
@@ -77,8 +77,9 @@ direction:
   name: Phase-Encoding Direction
   entity: dir
   description: |
-    The `dir-<label>` key/value can be used to distinguish
-    different phase-encoding directions.
+    The `dir-<label>` key/value can be set to an arbitrary alphanumeric label
+    (for example, `dir-LR` or `dir-AP`) to distinguish different phase-encoding
+    directions.
   format: label
 run:
   name: Run