Before I tried to solve this I did search the forum for the same question and saw the select face option.
So in my assembly I put on the select faces only filter. I then deleted my original decal and then reselected what I thought was the face of the subassembly (which is a container door), applied the decal, I then opened the container door and could still see the decal.
Looked through the options but not much of a clue. Would I be better to apply the decal to the container door subassembly itself rather than in the fine container assembly or better yet apply it to the part that is the door sheet?