We’ve added examples with serviceType (backport from R4). See https://data.developer.nhs.uk/ccri/ed/patient/1192/encounter
We’ve used a DCH CodeSystem for MainSpecialty/TreatmentFunction/ITK HL7v2 table 0069) which isn’t ideal.
I think this approach is OK, so long as there is an intent that the references are eventually resolvable (in an NHS timescale rather than a new software engineering concept that I invented). Nothing stopping them being backed up with the type/identifier as well so that can underpin search on other endpoints i.e. {base}/{type}?identifier={identifier}
I’m currently working on this. Have raised some comments on INTEROPen ryver Ryver
p.s. I think I’m going to go against my comments above and be more focused on a strict HL7v2 view of the world.