# Customers

A customer represents an end user (player) in your system. `externalUserId` is your merchant-side unique key: creating a customer again with the same `externalUserId` **updates** the existing record (create-or-update semantics).
When creating a [Cashier Session](#tag/Cashier-Session), you can identify the customer in one of three ways: an inline `customer` object, an `externalCustomerId` (your key), or an `internalCustomerId` (the OMNO customer ID returned here). `customerGroups` and `kyc.kycLevels` also drive PSP eligibility — see the Guides topic [Routing & PSP Eligibility](https://docs.omno.com/guides/reference/routing-and-psp-eligibility).

 - [POST /public/customer](https://docs.omno.com/apis/payments/openapi/customer/createorupdatecustomer.md): Creates or updates a customer for the authenticated merchant. Calling again with the same `externalUserId` updates the existing customer. `customerGroups` and `kyc.kycLevels` drive PSP eligibility — s