A physical chain-of-custody model keeps certified material or its sustainability characteristics connected to a physical supply chain. The exact connection depends on the scheme. Identity-preserved, segregated, and mass-balance models do not provide the same level of physical separation, so the model and its records must be named.
Book & Claim separates defined sustainability attributes from delivery of the physical fuel. The RSB Book & Claim Manual, for example, sets rules for eligible certified product, registration, transfer, and retirement of Book & Claim Units. A unit has meaning within that system because the manual, registry, account controls, and audit requirements define who may issue it and what the holder may claim.
A credible Book & Claim review should confirm the issuing scheme, eligible product and quantity, unique registry record, transfer history, retirement status, permitted claim wording, and safeguards against duplicate issuance or use. A certificate or spreadsheet outside those controls is not equivalent to a registry retirement statement.
Regulatory acceptance is a separate question. Whether an instrument supports FuelEU Maritime, another regulation, or a corporate disclosure depends on that rule and the evidence accepted by the relevant verifier or authority. A Book & Claim record should not be described as regulatory compliance unless the governing rule permits that use.
Verdaxis currently operates an order book for physical fuel bids and asks with supplier-declared product and sustainability fields. It does not issue, transfer, or retire Book & Claim units, and it does not check an external Book & Claim registry. A Verdaxis listing or trade record is therefore not evidence that an environmental attribute was retired.