2.1.1732 Part 1 Section, ST_RelationshipId (Explicit Relationship ID)

a.   The standard does not limit the length of the contents of the ST_RelationshipId simple type.

Office restricts the contents of this simple type to be at most 255 characters.

b.   The standard states that the values of the ST_RelationshipId simple type are a restriction on the XML Schema string datatype.

Office defines this simple type as being a restriction on the XML Schema id datatype.