Pinned Back supplies are where the pins come pre-inserted into the backs. Unpinned is where the backs and pins come as seperate items and you have to assemble them yourself before pressing the buttons.
Before covid, Pinned Backs were the norm. Almost always when you ordered button supplies, the backs would come with the pins already inserted. But since covid there has been lenghty lead time for pinning. So we have begun offering unpinned as an option so that you do not have to wait the ~8 weeks for pinning.