Reservations though is much like it sounds. You can reserve a villager currently in the town of the person that is cycling, and that means you most likely will get the villager you want. Only one person can reserve a villager, and reservations goes before lurking. Lets say I want to reserve Diana, for instance. When she is in boxes, the person that is cycling will write that in their thread, and also say reserver notified. They will decline any offers from anyone, unless I decide that I don't want Diana anymore, or that I can't take her in, or simply I took too long to answer. Then she will be up for the grabs, after a couple of hours.
Sorry if you knew this already, I just saw you asking what the difference was. In any case, happy to help!
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