The Join tool is used to simplify and organize your scene, by creating a single Decal object from a selection of Decals, in a non-destructive manner.
It is adviced to never use Blender's native Join tool to join Decals, as it's very destructive.
Any number of Decal objects
The Split tool then is used to split previously joined Decals apart again.
The Split tool only works for Decals joined using DECALmachine's Join tool, not for Decals joined via Blender's native Join.
Any number of previously joined Decal objects