LOINC panels (e.g. https://loinc.org/55418-8/) can include items that have "example UCUM units". When representing a LOINC panel as a Questionnaire, one would like to be able to offer these example units as part of an item of type quantity, but still allow them to enter their own unit, because the LOINC term does not require one of those units.

I propose a new extension for items of type quantity, "questionnaire-unitBindingStrength", whose values would be from the set defined at http://hl7.org/fhir/R4/valueset-binding-strength.html. The default value (if the extension was not present) would be "required", to preserve current behavior. For any other value, the user would be able enter alternative units to those specified via the unitOption and unitValueSet extensions. Potentially, clients could validate the entered unit to ensure that it was commensurate with one of the example units.

This approach would allow observationLinkPeriod/observationExtract to still work, allow the user to see what kinds of units are expected for given measure, and still give the user freedom to enter a different unit.