I have a udf that uses .ActiveSheet to identify the sheet calling it.
Works great as long as the sheet doing the calculating is the active
one.

This sheet with that udf has now been replicated - various what if
version copies. Each of these copies comes along with its own copy of
that udf in its cells. Question is, how does the udf ID the calling
sheet if it's not the currently active one?

There are global controls that change input values used by these
copied sheets. All must recalculate using this global value and their
own local variables.