For vital signs like BMI, height, weight, etc, there are standard codes defined by the core spec that SHALL be used by all conformant FHIR systems.  However, for other observations, conditions, etc., US Core constrains the code system, but there is lots of variability in how the codes are encoded.  We will add a section in HRex that talks about terminology and will point to it from CDex.  The section will specifically call out the fact that the same concept might be coded in multiple ways and that there are various ways of managing such searches.  We will talk with the EHR vendors to seek guidance around what the 'best' ways are to perform such queries (subsumption, value sets, enumerating the codes, text searches, etc.)