A short Internet search gave me this: "C" stands for countdown and "F" for finish. So "C" would be the time until you start your exercise. And "F" the time until you finish your set. Sounds somewhat plausible but I can't say if it really is correct...
I asked ChatGPT and it said it stood for Custom (set your own timing program) and Free (workout as you wish and press a button to record the times for later use). Not a bad idea but I had told it F is for the active period and C is for the resting period, so it is obviously wrong. Another wording gave me Focus / EFfort and Cool-down / Catch your breath.
I tried the Ecosia AI chat too, pasting the post title and it replied with Focus and Catch your breath.