(register ton) is a measure of space used to determine the size of a ship based on its interior. One R. corresponds to 100 cubic feet, i.e. 2.8316 cbm. The measurement of seagoing vessels is divided into gross tonnage (total content of the seaworthy enclosed interior) and net tonnage (content of the commercially usable space for cargo or passengers).
According to the 1994 Convention on Tonnage Measurement of Ships, gross registered tonnage (GRT) and net registered tonnage (NRT) are replaced by gross tonnage (GT) and net tonnage (NNT). The NRZ is the basis for determining port charges, canal passage charges or pilotage fees.

Source: logipedia / Fraunhofer IML