Language extension / "behavior"

Hello,

Section 1.6 Language Extensions of the standard says this:

A language extension is any documented implementation-defined behavior of a defined name in this standard that varies from the behavior described in this standard [...]

It is unclear to me if the term "behavior" could encompass additional keyword arguments passed to a function or macro. I'm assuming here that the rest of this section is honored (the additional arguments wouldn't alter the confirming code, and their presence is not explicitly prohibited).

WDYT?