Identifier datatype has a use element with datatype code, cardinality 0..1, and Required use of the IdentifierUse value set.  The IdentifierUse value set includes "usual" defined as The identifier recommended for display and use in real-world interactions, and "official" defined as The identifier considered to be most trusted for the identification of this item. Sometimes also known as "primary" and "main". The determination of "official" is subjective and implementation guides often provide additional guidelines for use.

In mapping ClinicalTrials.gov data to FHIR there is a common identifier (NCT and a string of numbers) this is the usual identifier, official identifier, primary identifier, and main identifier.  It is recommended for display and use in real-world interactions AND it is the most trusted for the identification of the item.

Which code is correct - usual or official?  Please revise the value set definitions to provide clarity for implementation.