# Update Reference NFT
A unique serial number is generated when a batch of carbon offsets are retired in a traditional registry such as Verra. This serial number contains important information pertaining the carbon offsets, which include the number of offsets, country, project identifier, vintage, certifying standard organization, and more.
For reference, the serial number generated by the Verra registry follows this format:
Next, you must update the empty *reference NFT* you minted on our dApp with the serial number generated by the source registry. This will create a permanent link between the off-chain registry and the on-chain Carbovalent registry. To recap, the retirement entry in the source registry (eg. Verra) must have the *reference NFT* token address and the *reference NFT* has the serial number embedded in the metadata.
**It is also to be noted that double counting/double bridging is now impossible as we use the serial number/token address to ensure it doesn't happen.**
**Note: some retirements, particularly those made through EMA, occasionally return multiple numerous serial numbers. Our dApp will allow you to input more than 1 serial number by clicking** `add additional serial number`**.**
Moreover, the serial number attached to the *reference NFT* metadata will be used to update the NFT with project specific information such as vintage, project identifier, source registry, and more. Some of this information will be stored on-chain, while the rest of it will be stored as metadata on [IPFS](https://ipfs.tech/).
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