#33944 proposes the addition for tx-resource, for passing up related resources when passing a value set as a parameter to a terminology server call. 

But there's a problem with that, which is that the value set may be large (e.g. an extensional value set with 1000s of included codes). And you might want to validate against the value set 1000s of times. This is a permutational explosion of network latency problems. And it's been an operational issue for the validator and IG publisher. 

To deal with this, the validator and IG publisher pass a session parameter to tx.fhir.org in the parameter 'cache-id'. When they do this, they remember which value sets etc they have already passed to tx.fhir.org under that session id, and expect tx.fhir.org to remember them and keep them in scope whenever operations using that session id are received. tx.fhir.org retains these resources for 30 mins after the last operation for the session is seen. 

the validator and IG publisher know to only do this if they see 'cache-id' in TerminologyCapabilities.expansion.parameter.

Note that this problem is not specific to the validator and IG publisher - it will occur anywhere where (a) value sets are being passed to the operations and (b) the operations are being called a lot.

We should document the use of the cache-id parameter for the $expand and $validate-code operations, and the TerminologyCapabilities. It's adoption should be optional for servers, but servers that do adopt it should pre-adopt for previous FHIR versions