Tonnage tax scheme: separate, option-based computation of shipping profits with tonnage income treated as business profits. The tonnage tax scheme requires companies operating qualifying ships to compute shipping profits separately under an option-based regime; companies operating owned or chartered-in vessels (including slot, space or joint charters) are within scope, while vessels chartered out on bareboat terms beyond three years are excluded. Tonnage income is computed as prescribed, deemed business profits, and the corresponding relevant shipping income is not otherwise chargeable; absent the option or coverage, shipping profits are computed under the Act's general provisions.