Sorry Herbert, this is probably going to annoy you, but the term "alpha layer" is incorrect and likely to confuse inexperienced readers.
Placing an extra layer on top of a single texture does not make it an "alpha masked texture" and doesn't create an extra channel (at least not in Photoshop). Layers, no matter how many and whether they are merged or not, don't create transparency effects (either masked or blended). To get transparency from a single image, you need to an extra channel. That's why it's called an alpha channel, not an alpha layer.