This is a standard Bitmap texture, also denoted as BML:

Most models have this, and is what you’ll be editing most of the time.

This is a Bitmap texture that uses semi-transparent materials, also denoted as BM:

This is the texture for the bottle used for potions. The red areas that are controlled by the alpha channel decide how translucent the texture is.

This is on a white to black scale (like most channels alpha is gray-scale).

This is the Alpha texture for that bottle isolated:

Gray is what you want for translucent like textures.

White = Solid

Black = So translucent that it is invisible.

You see these a lot on monsters, for example it’s how they display damage like scars on a monster.

Additionaly the player skins textures use the alpha layer for roughness (they do not have full RMT). (Thanks to Statyk for the information)