XML Schema Complete Reference, The

12.4. Oddball Datatypes

Several datatypes do not fit well into the foregoing partitioning:

• QName’s lexical space is related to that of Name, but QName values are not strings. QName is a “magic” datatype, in the sense of Section 12.3.3.

• boolean is the only mathematical datatype that is not numerical.

• hexBinary and base64Binary are both popular methods of encoding multiple-octet bit strings into printable ASCII character strings.

• anyURI is very much like a string-derived datatype, but uses a different mapping from its lexical space to its value space.

• notation is even more of an oddball: each application gets to define what a notation is. As far as XML is concerned, a notation is simply “something” that is identified by a system or public identifier. ...