Ah ok, good point. There will be situations where shelf marks refer to more than one title. However, then we still need to record each title against that shelf mark, and considering you can only link one title to each shelf mark per task, I think it still works as expected (unless I am completely misunderstanding)!
Thanks for pointing it out, we're sorting through our first batches of data at the moment and and there are various things to clean up, of course. So this might be another one to bare in mind.