Bass vs treble side tuner? Why bass or treble side of guitar machine heads / locking tuners?

Very simple: Imagine a 3l +3r guitar headstock like you find on classic singlecut guitars. Here you have 3 left guitar machine heads to which the bass strings are attached, which is why left guitar machine heads are also called bass side guitar machine heads. The treble strings are attached to the right three guitar machine heads, which is why these guitar machine heads are also called bass or treble side guitar machine heads.

The same goes for bass mechanics, of course!