Hot water storage vs usable

In my experience the actual capacity is the volume of the tank without any outlets cut in i.e. like a bucket, whereas the nominal capacity is the maximum with the holes cut in for inlet and overflow. The nominal is always therefore less than the actual. Good luck.

For vessels containing liquids, it is assumed to design that vessel with an extra volume on top of liquid to let a space for vaporizing of that liquid, to minimize rising up of pressure. This space varies from liquid to other and depends also on temp., and can reach up to 20% of actual liquid volume.