The 1st kind is when they cut down the entire first series of scripts and turn them into an hour and a half film. I've seen this a thousand times with movies like Porridge, Rising Damp E.T.C
The 2nd kind is when the film is seperate story or a sort of sequal to the last series. I hope this is what Red Dwarf is, I hope it picks off from where Series VIII left off.
