Types of References (5 to 7)
Authoritative definition (AD): The owner of this International Standard recognizes itself as an authoritative source for the definition of the concept, and not, for example, the label or code associated with the concept. A reference to a concept in another specification may be included if that concept is similar to the authoritatively defined concept in this International Standard.
Un-referenced definition (UD): This International Standard includes a concept for which there is no satisfactory authoritative, normative or qualified reference available. This International Standard defines the concept, even though the owner of this International Standard does not recognize itself as the authoritative source for such a concept. A reference to a concept in another specification may be included if that concept is similar to the un-referenced concept in this International Standard.� Other International Standards should not make an authoritative, normative or qualified reference to an un-referenced definition in this International Standard.
Non-referenced definition (ND): The definition of a concept in this International Standard is self-explanatory based on the dictionary meaning of the words that form the definition. Therefore no external reference is included.