Fx vs. Fxpak?

Hey there, I was wondering if anyone could help explain to me the difference between a standard fx file and an fxpak file? I’ve looked at lots of different skills and what fx.dbr files they are linked to, and (in my small brain at least) I can’t quite understand what dictates whether a skill is using a plain fx.dbr or an fxpak.dbr. Some skills also list an fx.dbr as many times as there are skill levels, while others only list an fx.dbr once. I assume for the skills with multiple fx.dbr per skill level that those are for effects that change as the skill is leveled? But then what is an fxpak for, and when would you use one over the other? Any insights into this or links to other topics discussing this would be very helpful. I did try searching the forums for info on fxpaks but I couldn’t find anything.