[ENH] Clarify meaning of raw vs derivative datasets

src/derivatives/introduction.md

@@ -4,9 +4,16 @@ Derivatives are outputs of common processing pipelines, capturing data and
 meta-data sufficient for a researcher to understand and (critically) reuse those
 outputs in subsequent processing.
 Standardizing derivatives is motivated by use cases where formalized
-machine-readable access to processed data enables higher level processing.
+machine-readable access to processed data enables higher-level processing.
+
+The following sections cover additions to and divergences from "raw" BIDS. Raw data are
+data that have been curated to BIDS, i.e. a BIDS “raw” dataset does not have another BIDS
+dataset as a source. If you are in doubt if your dataset is "raw" or
+is "derivatives" ask yourself the following question: "Are my data derived from at least
+one BIDS valid dataset?" if the answer is yes, then this is a BIDS derivative dataset.
+
+## Derivatives storage and folders structure
 
-The following sections cover additions to and divergences from "raw" BIDS.
 Placement and naming conventions for derived datasets are addressed in
 [Storage of derived datasets][storage], and dataset-level metadata is included
 in [Derived dataset and pipeline description][derived-dataset-description].