Alpha masks hide avatar parts 100% or 0%. They won't let you make the avatar translucent.
As these pictures show, the base avatar is hollow. The back sides of an open area aren't drawn, notice how the prim behind the masked eyes shows, not the inside back of the skull. Consider using prims to fill in the holes if you are making an avatar partially invisible.
The alpha mask is a wearable layer like the skin. An alpha mask layer has five textures. Head, upper body, lower body, eyes, and hair. The texture templates for alpha masks are the same ones used for texturing the avatar. And like other things using those templates, both arms are treated identically, and both feet. The skirt layer is not affected by alpha masks.
To make a custom alpha mask texture, make a texture that is partly transparent. Areas of the texture that are opaque will let the avatar show. Areas that are transparent hide those parts of the avatar. If all the textures that make up a mask are fully transparent, the avatar would be invisible.
To make your creations wearable with other items using masking, I suggest you include a copy of your mask texture(s) with your creation so the wearer can assemble a mask themselves if needed. Currently viewers only allow a user to wear a single alpha mask layer.
So when do you use a mask and when an invisiprim? Here are some differences to help you decide.
Masks use the same templates as avatar skinning. So both arms, hands, and feet will be treated identically. Invisiprims can be used to hide one arm and let the other show.
Mask do not interact with alpha textures or Linden water/trees/plants. Invisiprims do.
Invisiprims are prim wearables, and so can be adjusted by the wearable like any other prim attachment. Masks are a wearable layer and will fit every avatar the same.
Masks are a wearable layer and so do not add to your ARC. Invisiprims do.