The CQL Evaluation Result profile specifies that the dataAbsentReason extension can be used to indicate null evaluation results from operations. The default value set is left as-is, allowing for a variety of data absent reasons.

Are implementations expected to potentially provide different reasons for null values (as the value set implies)? I expect most engines aren't sophisticated enough to know or propagate a clear reason why a value is null – usually it just evaluated to null based on the rules of CQL evaluation.

If engines are expected or allowed to be specific about why something is null, then clarify that in the text. If, on the other hand, the intent is just to say "this is null", consider fixing the dataAbsentReason extension's valueCode to a single code, e.g.:

* parameter.value[x].extension[dataAbsentReason].valueCode = #unknown