He should study everything.
It never ceases to amaze that someone who is seeking instructor certification is trying to find the way to do the bare minimum study.
If it says "all," it applies to all.
He won't need to be tested on the fundamentals of instruction. He should pay attention to anything pertaining to the category sought, if adding a category; AIR for airplane, and all, which means applicable to all.
A very good practice is to shoot for something a bit higher than the bare minimum.