Net Tonnage
Modern equivalent
Gross tonnage minus engine, bunker and crew space
Gross tonnage less permitted deductions for space that cannot carry cargo.
Net tonnage is gross tonnage with the unearning parts taken out. Machinery spaces, bunkers, crew accommodation and navigation spaces are deducted, leaving the volume a ship can actually fill with paying cargo.
That is why it exists. Harbour dues, canal tolls and registration fees are assessed on net tonnage, so the deductions are not a courtesy but a matter of considerable money, and the rules governing which spaces may be deducted have been argued over for well more than a century.