I was told this came about because management was soliciting volunteers for credit instead of assigning OT. So if a staffing shortage is identified that would necessitate OT then they cannot offer credit any longer.
Say you are already volunteering to stay for two hours on the backend of a shift and then they decide they were short during the overlap you are fine. But if they decide they are short and holdover is required it must be OT.
I’m sure facilities interpret things differently like in all things ATC but I don’t know where you would find anything written down. It doesn’t appear to be in the slate book either the other credit info.