The SAPI DTD begins with a series of parameter entity definitions. A parameter entity definition defines a kind of macro that may be referenced and expanded elsewhere in the DTD. These macros may not appear in SAPI documents, only in the DTD. Other types of macros, called character references (see section Character References), may be used in the text of a SAPI document or within attribute values.

When a parameter entity is referred to by name in the DTD, it is expanded into a string.

A parameter entity definition begins with the keyword . Instances of parameter entities in the DTD begin with "%", then the parameter entity name, and followed by an optional ";".

The following example defines the string that the "%sample;" entity will expand to:

<!ENTITY % sample "var1 | var2 | var3">

The parameter entity expands to the string that may contain other parameter entity names. These names are expanded recursively.