You could convey the meaning here with about 3 sentences and it would be less confusing to implementers and far easier to understand. Try to be consise when possible.

Proposed Wording:

In many cases, the answer lists for questions will include concepts such as 'unknown' or 'not available'. In elements where these concepts are not part of the value set, the data-absent-reason extension can be used if necessary and appropriate.

(Comment 122 - imported by: Robert Dieterle)