The combination of images associated with UV maps is called "slots".
Selecting a Paint Slots or Canvas Image will also display the corresponding image in the Image Editor.
The slot system includes two painting modes:
This mode tries to detect the slots from the materials of the mesh.
For the Cycles renderer, all textures (Image Texture node) in the material's node tree are added in the slots tab.
- Active Paint Texture Index
A List view of slots. Activate a certain slot to use it for painting by LMB click on it.
- Single Image
You can just select an existing image and painting will use the active UV layer for painting.
Allows you to select the image used as a canvas.
Create a new image.
- UV Map
Allows you to select the UV layer for painting. (Same as the currently active UV map in the mesh's UV Maps panel.)
- Texture Filter Type
Set the interpolation mode of the texture. This can be Linear or Closest.
- Save All Images
Repack (or save if external file) all edited images. Same as in the Image Editor.
- Add Simple UVs
The Add Simple UVs does a simple cube unwrap followed by a pack operation. It's still recommended to make a custom unwrap.
This operator is available when the object does not already have a UV Map.