FHIR Conformance Layer include StructureDefinition resource, and I'm trying to understand if it is mandatory to provide anything there, when my server does not have any custom resources?
We are going to support multiple Implementation Guides (e.g. US Core and CarinBB), which have their own profiles and extensions. But all their StructureDefinitions are already definied on hl7.org and I can have links to those profiles from my CapabilityStatement and instances. So do I need to expose those structure definitions on my server?
Or it should be just empty, since I don't have anything custom?
Your CapabiltyStatement should declare a StructureDefinition for each resource you support that indicates what your actual system capabilities are - I.e. what data you can actually consume or produce. Typically this will involve a combination of the expectations of a variety of profiles as well as some additional stuff. You may have limits on repetitions, you might not support certain optional elements from some profiles, and might support some additional elements or extensions that none of the profiles expect support for. Very few implementations will have internal support that exactly matches an official published profile. However, if you do, you could technically point to that official profile rather than creating one of your own.