From the Assign rooms screen you can bind several physical rooms together to create a Virtual room.
The Virtual room can then be mapped against a new or existing Room type, and providing *all* of those rooms are free on a given date, the availability for that Room type is increased by 1.
The classic situation here would be a double and adjacent twin room with a lockable inter-connecting door. If both rooms are free, your staff could sell both rooms together as a Family Room.
Usually, the Virtual room will be mapped so it is only sold after the regular physical rooms of this type. If a booking comes in for that type and it gets assigned to the Virtual room, it will occupy all of the physical rooms it is made up of.
A Virtual room is indicated on the Assign rooms screen by a blue V.
Other applications could be selling all the rooms at the property for a wedding, or a corridor or floor for a group booking.