Flat head screws, are the ones with the cone shaped head, so they will self center in holes, and screw in flush. They are measured from the flat on top, to the tip of the screw.

Button head screws, are the "classical" screw head. Kinda dome shaped. They are measured from under the head, to the tip of the screw.

Cap head screws, are the ones with the round, barrel shaped top. These are measured from under the head, to the tip of the screw.

Often fasteners are listed with thread length, and shoulder bolts are a thing.

I have "tamiya specific screws" before the step screws because they seem to be a very tamiya specific thing. The low friction screws that tamiya sells are machine thread, instead of self tapping. So I felt the "m3" designation was.. still proper.