They will first fill up CW and try not to use First at all, But when closer to departure, in the final 72 hours, the final aircraft choice is made, which may or may not have a First cabin, and then they may start to allocate some CW to Flub. It's very difficult to give clear guidelines about this, and searching for "Flub" will throw up dozens of threads, other than to ask about it at the airport before departure. They tend not to sell First at late notice, even if theoretically they could/should.