Schema documentation: Guidelines for consistent application of the inheritance principle

[effigies]

Along with the increasing complexity of the schema, we're going to need some clear implementation guidelines.

We noted in the schema sprint documents, that we need clear test for determining associations via the inheritance principle. For example, a _physio.tsv.gz file may be associated with a _bold.nii file while having an entity (recording-<label>) that is not permitted for _bold.nii.

Here is a first pass of text:

Two files are associated based on the intersection of their allowable (REQUIRED + OPTIONAL) entities PLUS data type. A "secondary file" (not a well-defined term) is associated to a "primary file" if every common (i.e., in this intersected set) entity in the secondary filename occurs in the primary filename.