Extensible select and enumeration types

and their elements in schema Verification_and_validation_arm taking into account BASED_ON types


 identification_item - EXTENSIBLE SELECT
          Requirement
          Requirement_version
          Requirement_view_definition

 requirement_assignment_item - EXTENSIBLE SELECT

 requirement_source_item - EXTENSIBLE SELECT

 verification_evidence_item - EXTENSIBLE GENERIC_ENTITY SELECT