Qs -- *ist custom functions

Hello all -- occasional lurker from the US here.

I very recently bought a *ist (film, of course!) with the FA-J 28-80 lens. Lovely camera, and it's great fun learning to use it.

I've used Pentax for years and years -- I still have a Pentax Spotmatic I bought new in 1969, but this is my first modern camera with all the doo-dads.

My question is: is there a good written source (printed or online) explaining the custom functions in the *ist? I know the manual describes the *what* but I want to learn more about the *why*! Since I am used to far simpler cameras (adjust shutter and speed and focus, then snap) I am somewhat bewildered by the choices.

Some are self-explanatory, no problem, but then there's #6, for example, "superimpose focus points." What does that mean, and what are the pros and cons of the two options?

I am eager to learn, and would just like someone to point me in the right direction. Thanks!