Per today's NIDM-Terms call, one useful step toward integrating NIDM BIDS-Terms and the BIDS schema would be to divorce the layout rules (e.g., entity order in entities.yaml) from the definitions of the objects (e.g., the actual entities, their descriptions, etc.).

At the moment, I'm thinking something like this:

schema/
    entities.yaml: just a list of entities in order
    objects/
        sub.yaml: definition of the sub entity
        ses.yaml: definition of the ses entity
        ...
        split.yaml: definition of the split entity