The US Core Tribal Affiliation extension does not align with the C-CDA definition which explicitly excludes enrollment. Therefore, its not clear why the optional element isEnrolled is included in the Tribal Affiliation extension other than it might be there to align with the existing eCR extension. 

Some differences noted between the eCR extension and the US Core Extension:

• US Core Data element name: tribalAffiliation vs: TribeName;
• US Core value datatype: CodeableConcept vs: coding;
• US Core data element name: isEnrolled vs: EnrolledTribeMember;
• US Core isEnrolled is not MS, EnrolledTribeMember is MS and the link to the bound value set in the eCR extension is to and outdated version of THO. 

Link to eCR extension: http://hl7.org/fhir/us/ecr/STU2.1/StructureDefinition-us-ph-tribal-affiliation-extension.html 

There should be explanatory text added to explain the reason for the difference - especially since C-CDA specifically excludes enrollment. Understand that at some point in the future the eCR extension will be aligned with US Core. Its possible eCR does not need the enrollment data element.