Disclaimer : All my stabillity training is about surface ships.
Reserve buoyancy is the volume of the vessel that could be buoyant, but isn't, since it's above the waterline. (IE, accomodations, focstle store, etc). So a submarine with a lot or reserve buoyancy would be one ridding very high in the water when surfaced.
It'd take more ballast to submerge one such submarine, because of the whole Archimede thing (Essentially, to have neutral buoyancy, your submarine has to weight as much as its entire volume of water weight. Less than that, it's going up, more than that, it's going down)
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