[ADD] Parametrically linked file collections

- Description

src/02-common-principles.md

@@ -161,6 +161,22 @@ 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.
 
+### Parametrically linked file collections
+
+A set of files that are related to each other based on a repetitive acquisition of
+sequential data by changing acquisition parameters one (or multiple) at a time.
+The suffix serves as an identifier for these collections, holding a logical grouping
+of constituent files. 
+Within each collection, files MUST be distinguished from each other by at least one 
+entity (e.g., `echo`) that corresponds to an altered acquisition parameter (e.g., 
+`EchoTime`). Note that these entities MUST be described by the specification and the
+parameter changes they declare SHOULD NOT invalidate the definition of the accompanying
+suffix. For example, the use of the `echo` entity along with the `T1w` suffix casts doubt on
+the validity of the identified contrast weighting.  
+Provided the conditions above are satisfied, any suffix (e.g. `bold`) can identify a parametrically linked file collection, although certain suffixes are exclusive for this purpose (e.g., `MP2RAGE`).
+All the use cases concerning this convention are RECOMMENDED to be included in the [file collections]() appendix for clarity. This convention is mainly intended for but not
+limited to MRI modalities.
+
 ## Source vs. raw vs. derived data
 
 BIDS was originally designed to describe and apply consistent naming conventions