> For the complete documentation index, see [llms.txt](https://docs.t-rex.network/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.t-rex.network/the-primary-register.md).

# The primary register

T-REX is organized around one idea: the on-chain contract holds the register of the asset and enforces it. It is designed to be the primary register, not a copy of a register kept elsewhere. Everything else in the protocol follows from it. The identity layer exists because a register must know its holders. The compliance layer exists because a register must refuse what it may not record. The cross-chain architecture exists because a register must see the whole distribution of the asset.

Whether the on-chain register is also the legal register of a given asset depends on the jurisdiction and on the asset. Where the legal register has to be kept off-chain, it replicates what happens on T-REX. The first page of this section covers both cases.

This section explains the idea and its consequences.

| Page                                                                                                           | What it covers                                                                                                                                                                            |
| -------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| [From digital twin to primary register](/the-primary-register/from-digital-twin-to-primary-register.md)        | The reconciliation problem, why a token that mirrors an off-chain book does not solve it, what changes when the on-chain record leads, and how that record relates to the legal register. |
| [The register as an on-chain CSD](/the-primary-register/the-register-as-an-on-chain-csd.md)                    | The functions of a securities depository and the contracts that perform them. Why a register must be global to enforce the rules of a security.                                           |
| [T-REX and ERC-3643](/the-primary-register/t-rex-and-erc-3643.md)                                              | What the standard defines, what T-REX adds around it, and where T-REX narrows it.                                                                                                         |
| [Eligibility and compliance: the two gates](/the-primary-register/eligibility-and-compliance-the-two-gates.md) | What each gate checks, how it decides, and which operations pass through it.                                                                                                              |
| [Technical finality and legal finality](/the-primary-register/technical-finality-and-legal-finality.md)        | What is final on a satellite, what is recorded on the register, when a transfer is legally final, and how the protocol manages the time in between.                                       |


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