Is it correct to call any surface a face because it is part of a polysurface?
In scripting/programming terms at least, yes - a polysurface is a Brep and each component is a “face”.
–Mitch
It is actually correct even for what at the top level is just a single surface, since that is held as a one-face brep. I guess if the face is not trimmed, then for most purposes differentiating is kind of formality that keeps things consistent under the hood, but a trimmed face is different from the underlying surface,
-Pascal