Specific guidance on how and when to use SFCU is being added to the GHP IG.  After much discussion, our approach to the FHIR base spec is to be fairly vague on guidance, and instead refer to the GHP IG for guidance, so that we might iterate on that guidance and resolve differing perspectives in a venue that is more easily updated.   There are two documents we are drafting: Updates to the FHIR specification to defer to the GHP IG:   https://docs.google.com/document/d/1ne73sA_4pQvmFy1w9FgGg1Z8M6zwCkryuC96vDjsMcw/edit Updates to SFCU and guidance: https://docs.google.com/document/d/16n4a4xs7Vm9ri4s_HsvPqfjvkpQ8KnRN/edit   As this tracker is against the FHIR core spec, the resolution here will simply be that we will include language that defers this guidance to the GHP IG (described in the link in #1 above).  The actual answer to your question may be addressed in the GHP IG, though likely on different timelines than the FHIR R5 publication cycle.  Also, determining to which entity contextual extensions on Observation apply is a discussion for the owner of the Observation resource (i.e., O&O).