Opinion: Composition makes more sense than a List: A set of healthcare-related information that is assembled together into a single logical package that provides a single coherent statement of meaning, establishes its own context and that has clinical attestation with regard to who is making the statement. For example, when a doctor signs on something, he/she signs on the document, not on each line of the prescription. In general, we are replacing a single "real life" document here. easier for the client to edit the list What exactly do you mean by that? Also, who/what is "the client" here? Does the List have "medications" code? unnecessary complexities. Can you please elaborate? If a prescription is stored in a FHIR server, with the currently suggested data model, what is the search to be performed to find all prescriptions on that server that belong to this pilot/project?

I think that each organization should define appropriate FHIR profile. If you (whatever 3rd party) need to communicate with that organization, you must support the profile. You should not have arbitrary validations which are not defined anywhere.