This position paper proposes a unified approach to "additionality" in the deep-sea shipping sector, focusing on enhancing consistency and upholding integrity within the book-and-claim market. Book and claim is a system that allows emissions reductions or sustainable actions, such as using low-carbon fuels, to be credited and tracked within the same value-chain even if they occur separately from where the fuel is consumed. By establishing a shared interpretation of additionality aligned with shipping regulations, this paper aims to create a level playing field among market actors—including registries, issuers, buyers, and verifiers—while recognising that criteria for additionality may differ as the market evolves.