Value Set Extension valueset-unclosed satisfies an important requirement to identify a value set expansion that does not include all concepts that would be expected from evaluating the value set definition. An incomplete value set expansion used in an implementation could impact patient safety.

On the August 12, 2019 Vocab Value Set Expansion call, two approaches were proposed (with a preference for #1) to ensure the implementer community is aware of the importance of this element and it is valued appropriately.

1. Promote the valueset-unclosed extension to core. Requires: connectathon testing. (assumption: it should be simple for a terminology service to identify when input parameters have altered the expected expansion list)

2a. Update section 4.9.8 Value Set Expansion https://www.hl7.org/fhir/valueset.html to require/strongly suggest that the valueset-unclosed extension be valued.

2b. Update the extension description and constraints for https://www.hl7.org/fhir/extension-valueset-unclosed.html to reference a more common use case such as technical limitations (as seen in many FHIR value set expansions - "First 1000 codes" displayed message) Suggested text: Marks that the expansion is incomplete, and values other than those listed may be valid. This may be used when technical limitations prevent a full expansion, or when post-coordinated codes are allowed, and no practical expansion can be produced.