There are several times that a list of items must be arranged linked to several containers. A container may have many items, but an item may be listed just in one container. So a one-to-one relation doesn' fit, for it doesn't allow more items into the container. Neither do a one-to-many relation, for each item is not 'disabled from being selected' after linked to a container.
There must be a 'many-to-one' relation, where you have a set of origin fields (the items) and can distribute them between some prime-tier fields (the containers). It's just like putting stuff into a bag/box/drawer to 'clean'.
That would even allow for proper DB nesting into each other, creating tree libraries.
It's like probability without replacement.
eg- I may make a DB for a food factory and thus have a library with fish, meat, vegetables. Another library with beef stick, carrot, tuna, chopped pork, onion. I want to arrange the second one into the first. So on meat field, I pick chopped pork and beef stick to link. When I edit fish field, I cannot pick (indeed they don't even show up on the list) chopped pork nor beef stick, for they are already linked.