When you receive a booking for an accommodation, the booking is automatically assigned to an available room.
So if John Doe books Double Room he gets automatically assigned to an Double Room that is available for his entires stay. E.g. here Doe is booked in the Double Room 105 for three days.
However, I would rather like to have Doe in room 106, since it has an ocean view. To move the booking to room 106, select the gray up and down arrow in front of his name and drag it to room 106.
As long as there is availability on room 106 for three days to booking can be moved.