In the Cambo SC system and successors, many of the attachments are modified lens boards, or, if you like to think of it that way, attach to the standards like lens boards. Bellows frames, backs, lens board adapters, ...
Graflok backs accept insertion type film holders (sheet film holders, some roll holders) that go between the focusing panel and back. The focusing panel is removed to allow clip-on type roll holders to be attached. Without a focusing panel a Graflok back is useless. Can't use a sheet film holder, can't focus so there's no point attaching a clip-on type roll holder.