Actualist philosophy concerns itself with what will actually happen, while possibilist philosophy only requires knowledge about what could hypothetically happen. The latter holds even if we are certain a supposed possibility will not occur.
To concretise this let us take a student who should study for the next day’s exam but has been invited out. For the sake of the thought experiment we need to buy into counterfactual determinism, which is to say that given the student’s choice we know how she will act, not just how she could act.
If she chooses to stay and “study” we know she will lose motivation and binge-watch Rick & Morty on Netflix.
With this set up a possibilist would argue that she should stay in her room, even if she fails to study, because going out to have fun would be the wrong this to do.
An actualist would advise the student to go out with her friends because she wasn’t going to study anyway, even if it is in some sense a possible option.