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The Select tool is used to quickly select all decals of one or more parent objects.
It's simimlar to Blender's object.select_grouped(type='CHILDREN_RECURSIVE', however, will only select Decals, instead of all children.

Check out this part of the Preferences video, where the tool and keymap are explained.


Any number of objects carrying Decals.


For multiple objects, you want to double click on the background, otherwise you will deselect one of the objects.
Also, for users of the 2.80 default keymap, I suggest changing the keymap to something else, because selecting on the background will deselect all objects.


Keep Decal Parents Selected

Adds the Decal objects to the already selected parent objects.