Tonnage determination by statutory certificates ensures objective tonnage income computation and limits administrative discretion, aligning with international practice.
The net tonnage for tonnage income must be determined from prescribed certificates: Indian ships by Merchant Shipping Rules or the 1969 Convention certificate as applicable; foreign ships by a DG Shipping licence reflecting Flag State tonnage certificates or other evidence acceptable to the DG; inland vessels by Inland Vessels Act, 2021 certificates. Reliance on statutory certificates is central, reducing subjective measurement and constraining administrative assessment to verification of certificate authenticity.

Allocation of tonnage income: proportional or independent computation affects tax treatment of jointly operated qualifying ships.
Computation of tonnage income for jointly operated qualifying ships follows a two-step approach: where participating companies' shares are definite and ascertainable, income is allocated proportionately to each company; where shares are not definite and ascertainable, tonnage income for each operator is computed as if it were the sole operator. The rule aligns taxation with economic interest, creates documentary and compliance incentives, functions as an anti-avoidance measure, and may interact with cross-border tax rules, requiring clearer guidance on "definite and ascertainable" shares and documentation standards.

Tonnage tax regime: ships' taxable income computed by daily tonnage rates and aggregation, excluding deductions.
Clause 227(1)-(6) prescribes a ship wise tonnage tax: each qualifying ship's tonnage income equals its daily tonnage income multiplied by qualifying days, with daily rates set by a four tier slab linked to certified net tonnage. Tonnage includes certified physical tonnage and prescribed deemed tonnage for slot and sharing arrangements, rounded to the nearest hundred tons. A non obstante clause bars any deductions or set offs, making the computed tonnage income the exclusive tax base under the Part.