The CodeableConcept constraint:

Constraints

ccc-2: Only one coding in a set can be chosen directly by the user (xpath: count(f:coding[f:primary/@value='true'])<=1)

The context of use usually makes rules about what codes and systems are allowed or required in a particular context by binding the element to a value set.

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This is not clear to me. Specifically I don't understand what "Only one coding in a set can be chosen directly by the user" is supposed to mean. From what I read here CodeableConcept can have multiple Codings, which makes sense. So is the "set" the unordered set of codings for this CodeableConcept? I'm lost so this needs more clarity to be useful.