Learning texture painting mask with the help of Blender official User Manual... nah... just kidding ;-)
In the Texture Paint mode, "painting slots" are displayed in the tool shelf of the 3D editor (1). Entries seems to be images (3) associated with material textures (2), provided they have a UV mapping associated with.
If this is the case, is there a reason for having them listed at two locations under two different names?
I see a possibility to associate a different painting mask to each "painting slot" (masking is actually the topic I'm trying to understand). But as it would be possible to have this mask defined in the Property editor without adding another concept, I'm a bit perplexed...
I've read this page, but it doesn't help for this question.