I think maybe you’re talking about a shadow catcher?
Maybe if you describe a bit more what you’re trying to do…cause it’s not real clear what result you want to get. You can remove objects from your render in post (using an “object index” pass), leaving only their shadows, but I’m guessing that might not be what you really want.
I agree, more information about the scene would help. I was thinking a shadow catcher as well, but they can also act as a mask. If there’s an object in the scene that’s behind it, or partially obscured by it, then it would be masked by it. Strangely, I used one once and mistakenly intersected the shadow catcher with a model (I was using it to mask the model I wanted it to appear later in an animation), and the model behind it was completely visible. Of course the alphas in the shader on the object may have confused Carrara as well.