SGML is a system for defining markup languages. It allows authors to mark up their documents by representing structural, presentational, and semantic information alongside content (see also Overview of SGML Resources).

SAPI is one of the examples of a SGML markup language. SAPI is compatible with XML. SAPI definitions are provided in this document with the SGML syntax.

Names of SAPI elements mostly correspond to related XSL objects (see also The Extensible Stylesheet Language Family (XSL)).