TBD: See Normal map for examples.
If no alpha channel is specified, the diffuse map will default to a full white alpha channel.
The alpha channel in the diffuse map has a context-sensitive interpretation. In gloss and bump-gloss materials, the alpha channel indicates a blend between the gloss reflection (black alpha) and the diffuse texture color (white alpha).
In most other materials, the alpha channel is used as an opacity control with black being full transparency and white being fully opaque. This is referred to as "alpha blending". As a special case of this technique, a diffuse texture may have its entire alpha channel set to either pure black or pure white, with no grays used. This specialization is often referred to as "alpha masking".