2.2.3 [ECMA-262-1999] Section 10.1.1, Function Objects
Internal functions are built-in objects of the language, such as parseInt and Math.exp. An implementation may also provide implementation-dependent internal functions that are not described in this specification. These functions do not contain executable code defined by the ECMAScript grammar, so they are excluded from this discussion of execution contexts.
In the above paragraph the phrase "internal function" is actually being used as a synonym for "built-in objects" (as defined in [ECMA-262-1999] section 4.3.7) that are functions. The implementation-dependent built-in functions provided by JScript 5.x are described in [ECMA-262-1999] section 15.