Well, okay, that’s my bad. (Actually, as I said at the very beginning, it just popped in my mind. )
We can suppose some scenarios:

I’m trying to locate all cats and dogs in one image only with bboxes using faster-rcnn.
I’m trying to segment all cars and persons in one image with masks using deeplabv3.
I"m working on an instance segmentation using mrcnn.
Really grateful to your patience.