RF: reword/shorten "Participant names and other labels" referring to …

…<label>s and <index>es

src/02-common-principles.md

@@ -339,15 +339,13 @@ Example:
 
 ## Participant names and other labels
 
-BIDS uses custom user-defined labels in several situations (naming of
-participants, sessions, acquisition schemes, etc.) Labels are strings and MUST
-only consist of letters (lower or upper case) and/or numbers. If numbers are
-used we RECOMMEND zero padding (e.g., `01` instead of `1` if you have more than
-nine subjects) to make alphabetical sorting more intuitive. Please note that the
-sub- prefix is not part of the subject label, but must be included in file names
-(similarly to other key names). In contrast to other labels, run and echo labels
-MUST be integers. Those labels MAY include zero padding, but this is NOT
-RECOMMENDED to maintain their uniqueness.
+BIDS allows for custom user-defined `<label>`s and `<index>`es e.g., for
+naming of participants, sessions, acquisition schemes, etc. Note that they
+MUST use only allowed characters as described in [Definitions](02-common-principles.md#definitions)
+above. In `<index>`es we RECOMMEND zero padding (e.g., `01` instead of `1`
+if you have more than nine subjects) to make alphabetical sorting more
+intuitive. Zero padding is NOT RECOMMENDED to be used to maintain their
+uniqueness.
 
 ## Units