noun
tonnage
Refers to the capacity or weight of a ship measured in tons, or the total amount of cargo in tons.
この船の噸数は5万トンです。
The tonnage of this ship is 50,000 tons.
トン is the unit 'ton', while 噸数 refers to the measurement of tonnage.
噸 is a kokuji (Japanese-created kanji) for 'ton', combined with 数 (number/amount).