The current definition of structuredefinition.type is a code, with extensible binding: The type this structure describes. If the derivation kind is 'specialization' then this is the master definition for a type, and there is always one of these (a data type, an extension, a resource, including abstract ones). Otherwise the structure definition is a constraint on the stated type (and in this case, the type cannot be an abstract type).

This definition/value domain does not work for Logical models. The type should be url, with the same extensible binding, and a note that the default base URL is http://hl7.org/fhir/types. Logical models that use different types have to provide a url reference that - the logical model canonical URL.

This leaves the existing data element content unchanged, but allows for logical models