Using your analogy, If you have a PNG that has an alpha channel it defaults to total white(full opacity). You can delete the alpha so that the PNG has no alpha channel. You can leave the alpha channel just as it is, but sometime you need to make adjustments to the alpha channel. just like how an alpha channel has to have a value for every pixel of the image, UV Maps must have every vertex, edge, face, vertex normal, split normal(Also known as edge normal), and face normal accounted for in it’s mapping. If you take an image that doesn’t have an alpha channel and then add an alpha channel to it in Photoshop you will get asked do you want to default to full opacity(White) or full alpha(Black) or at least it use too. I haven’t used Photoshop since version 7.0 on windows 98. Gimp just defaults to full opacity. If you add another UV Map or delete the UV Map and add one a default UV Map will be created because you CAN NOT have a blank UV map. Again ALL vertices, edges, faces, vertex normals, split normals, and face normals have to be mapped.
