Reservations are automatically assigned to the first available room/apartment, within a certain type. For example, if you have 3 studio apartments 101, 102, and 103, and all 3 are available, the reservation is automatically assigned to studio 101. Of course, you can then manually transfer the guest to whichever apartment suits you with a simple drag-n-drop calendar or changing room type in the reservation edit screen.

Check here how a room is allocated: http://helpdesk.otasync.me/en/articles/6092122-how-is-a-room-allocated-to-a-reservation
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