Reservations are automatically assigned to the first available unit of the reserved room type.
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