You mean render elements, right? You would like to render your image and have decals as masks so that you could for example select a specific decal in a 2D editor?
Exactly! In my case, I have used a decal to create crossing lines for pedestrians in a road. The road is the main object with an asphalt as a material on it and the decal is the crossing lines. I wanted to lower down the exposure of that crossing lines in Photoshop but the selection on anything else except Monochrome or RGB masks doesn't give me a detailed mask. As a result the lower exposure adjustment creates a frindge-like effect around the crossing lines.