What is a paint slot (compared to an image with a UV mapping)? 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...

Using 2.79.



I've read this page, but it doesn't help for this question.
A texture painting slot is how you can control which is the active target for paint without having the node editor open to hunt and find that corresponding image node to click it to make it active, nor having to find the corresponding texture in the material texture properties.

The origin of the texture slot panel was actually a GSOC adaptation of the Texture Paint Layer add-on that offered similar functionality in the N Panel, and it also gave the ability to not need the properties editor to be open to determine which image texture was to be the target.

Masking is much more than the Stencil type that is normally used between image textures, and the actual mask tool is very powerful and I have explored it a few times on my youtube channel. You can utilize a different UV projection for masking, or you can use the same as the existing UV mapping for the target image.