ENH: Clarify preference for approximate/undefined values

src/02-common-principles.md

@@ -130,17 +130,17 @@ specification.
 BIDS specifies that metadata be stored in human-readable formats such as
 TSV (see [Tabular files](#tabular-files)) or
 JSON (see [Key/value files](#keyvalue-files-dictionaries)).
-In some cases, this duplicates metadata internal to the data file.
+In some cases, this duplicates metadata contained in a data file.
 
 In cases of conflict, the BIDS metadata is considered authoritative.
 If BIDS metadata is defined,
 format-specific metadata MUST NOT conflict to the extent permitted by the format.
 
 If an exact representation of the BIDS metadata is possible in the format,
 then the metadata SHOULD be duplicated inside the data file.
-If an exact representation of BIDS metadata is not possible in the format,
-then the data file metadata SHOULD be undefined
-or set to the closest possible approximation of the BIDS metadata.
+If an exact representation of the BIDS metadata is not possible in the format,
+then the closest possible approximation of the BIDS metadata SHOULD be used.
+Special null or undefined values MAY be used when available.
 If the format-specific metadata field is defined,
 the BIDS metadata SHOULD also be defined.