The special data type is a string conforming to a case-specific standard or constraints. The constraints on each special data type instance are either unique to that structure type or are not simply expressed. For example, the payload of an IDNO structure may obey different rules for each possible TYPE substructure.
Each special data type is distinct. The URI for the generic data type subsuming all Special data types is xsd:string (the same as the Text data type).