Representation of extended value sets

Representation of extended value sets - How does the model in section 5.3 account for CWE value sets? From the Data Type specification, "… for CWE domains the local code is a valid member of the domain…", whereas in the CTS model, all codes in a value set must be drawn from a single code system. It may be necessary at the very least to add a ValueSet.extensible attribute. Within an implementation there may be the need to reflect the CodeSystem from which the extensions are drawn.