Proposed Wording: Implementations processing the data in resources SHALL check for modifiers anywhere they may appear, and if a modifier extension is present, SHALL do one of these things:

understand the impact of the extension when using the data

refuse to process the data

carry a warning concerning the data along with any action or output that results from processing the data to inform users that it has not fully understood the source information

Comment:

What http response (e.g. 409?) would be returned by "create" or "updated" if an modifier extension is detected. Should it part server validation it should check for these?

Some clarification in "2.14.0.1.2 System Profiles" would be useful to indicate to server implementers that they need to be aware of "unrecognized" modifier extensions?