Reserve buoyancy can be defined as the volume of the enclosed spaces above the waterline. It can be expressed as a volume or as a percentage of the total volume of the vessel.
The volume of the enclosed area above the waterline are not providing buoyancy but are being held in reserve. If some extra weights are loaded to increase the displacement, these spaces above the waterline are there to provide the extra buoyancy required.
Enclosed spaces, which provide buoyancy in addition to that required by a vessel to float. It is always considered in the assignment of freeboard to a ship.