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.