When we took on the challenge of building Asia's first SAF credit registry — the platform that became globalsaf.co — we thought we understood book-and-claim. We had read the ICAO CORSIA documentation, studied the RSB chain-of-custody standards, and talked to enough airlines to know the market need. But building a registry from scratch exposed a level of complexity that no whitepaper captures.
This is what we learned about how book-and-claim actually works in practice, the safeguards required to prevent double-counting, and why getting the data model right matters more than anything else.
The Book-and-Claim Mechanism: What It Really Means
Book-and-claim exists because SAF cannot be in every fuel tank at every airport. Production is concentrated in specific regions, supply chains are constrained, and blending infrastructure is limited. Without book-and-claim, an airline could only claim emissions reductions at airports where SAF is physically delivered — which would cripple adoption.
The mechanism decouples the physical fuel flow from the environmental attribute. A producer in Rotterdam blends SAF into the fuel supply. The environmental benefit — the lifecycle emissions reduction compared to conventional jet fuel — is captured as a credit. That credit is registered, transferred, and eventually retired by an airline that may be operating flights out of Changi. The physical fuel goes one way. The credit goes another.
The registry is the trust layer. Without it, there is no way to guarantee that the same environmental benefit is not sold twice, claimed by two different airlines, or counted against two different compliance obligations. Every credit must have a unique lifecycle: issuance, transfer, retirement. No exceptions, no manual overrides.
CORSIA Alignment and RSB Verification
ICAO's Carbon Offsetting and Reduction Scheme for International Aviation — CORSIA — sets the global baseline for how SAF credits can be used to meet compliance obligations. Our registry had to align with CORSIA Eligible Fuels (CEF) criteria, which means every credit in the system must trace back to a fuel that meets specific sustainability criteria, lifecycle emissions thresholds, and certification requirements.
We chose to build the verification layer around the Roundtable on Sustainable Biomaterials (RSB) standard, which is one of the CORSIA-approved sustainability certification schemes. In practice, this means the registry ingests RSB certificates and cross-references them against production batch data. The system verifies feedstock type, production pathway, and the resulting lifecycle emissions value before any credit is issued.
The hardest part was not the verification logic — it was the data ingestion. Producers, blenders, and certification bodies all use different formats, different identifiers, and different reporting cadences. We built a normalization layer that maps heterogeneous inputs to a single canonical data model. Every credit in the registry has a fully traceable lineage from feedstock origin through production, blending, and certification.
Zero Double-Counting: The Non-Negotiable Constraint
Double-counting is the existential risk for any environmental credit system. It comes in three forms: double issuance (the same batch of SAF generates credits in two registries), double claiming (two entities retire the same credit), and double use (a credit is used against both a compliance obligation and a voluntary commitment).
We addressed each form with specific technical safeguards. For double issuance, every credit is linked to a unique production batch identifier that is checked against a global deduplication index before issuance. For double claiming, credits use a state machine model — once a credit moves to 'retired' status, it is cryptographically sealed and cannot be transferred, split, or reactivated. For double use, the retirement record includes a purpose code that locks the credit to a specific compliance framework (CORSIA, EU ETS, or voluntary).
We also built a reconciliation process with other registries. As the market matures and multiple registries operate in parallel, cross-registry deduplication becomes critical. Our system exposes a verification API that allows external parties to confirm whether a specific production batch has already been registered, without revealing commercially sensitive transaction data.
Chain of Custody: From Producer to Retirement
The chain of custody is the audit trail that connects every credit back to the physical SAF it represents. In our registry, this chain has five mandatory links: production (who made the SAF, from what feedstock, using what pathway), certification (which body verified it, under what standard), blending (where was it introduced into the fuel supply, at what ratio), issuance (when was the credit created, with what attributes), and retirement (who claimed it, for what purpose, against what flights).
Each link is immutable once confirmed. We use an append-only event log as the underlying data store for custody records. Corrections are handled by issuing adjustment events that reference the original record — the original is never modified or deleted. This gives auditors and regulators a complete, tamper-evident history of every credit.
One thing we underestimated was the importance of partial credit handling. A single production batch might generate credits that are split across multiple buyers. The registry must track fractional ownership while maintaining the integrity of the chain of custody for each fraction. We ended up modeling credits as divisible units with parent-child relationships, where the sum of children must always equal the parent.
Why This Matters Now: Singapore's 2026 SAF Mandate
Singapore has announced that it will mandate a 1% SAF blend for all flights departing Changi from 2026. This is not voluntary. Airlines operating out of Singapore will need to demonstrate compliance, and book-and-claim through a recognized registry is one of the accepted pathways.
The mandate means the demand for reliable, auditable SAF registries is about to become urgent. Airlines that wait until the requirement takes effect will be scrambling. The ones that engage now — understanding how credits are issued, transferred, and retired — will have a structural advantage in both compliance cost and operational readiness.
We built this registry because the market needed infrastructure, not just ambition. The book-and-claim mechanism is elegant in theory and demanding in execution. Getting it right is the difference between a credible decarbonization pathway and a greenwashing liability.