This is relevant for building asparagus staging rockets. Basically each asparagus-stage should be ejected separately, so they should be on a separate stage, but for consistency and quick building they should not be constructed individually (I have found the need for many separatrons on my large asparagus fuel tanks, so placing an array of separatrons on each pair of asparagus stages is tedious).
I can of course use the alt-key to copy the stage, which takes me most of the way there, but I still have to reorganize all my separatrons and decouplers to put them onto a new stage. How can I "target" my copied rocket section so it will place its items into a new/different stage?