> For the complete index of the 402pay docs, see [llms.txt](https://developer.402pay.co/llms.txt).

# Core concepts

Payments, checkouts, links, customers, events and your wallet, and how they fit together.

## How a payment flows

1. 01 You ask for money Your server creates a **payment**, or you share a **payment link**.
1. 02 The customer opens checkout Each attempt is a **checkout**. For crypto, it reserves a fresh address in your wallet and locks the rate.
1. 03 Funds arrive A transfer lands on chain, or a card payment is approved. Checkout follows it until it confirms.
1. 04 You hear about it The payment succeeds, an **event** is recorded, and your webhook endpoint gets it.
1. 05 It lands in your wallet The money is a transaction in your **wallet**, and the payer is a **customer** you can look up.

## Businesses

A business is one merchant account, with its own wallet, keys, payments and settings, and one person can own several. A secret key belongs to one business and acts only as it. Each business has exactly one wallet, which receives every payment. IDs start with `biz_`.

[Authentication](https://developer.402pay.co/authentication.md)[Security best practices](https://developer.402pay.co/guides/security.md)

## Payments

A payment is money you're owed for one thing, at an exact amount in a currency you choose, with a status that moves from `pending` to `succeeded` and room for your own `metadata`. One you create through the API has a hosted checkout at its `url` and your own `reference`. One made through a link has its `link_id` instead, and a `url` of `null`. IDs start with `pmt_`.

[Accept a payment](https://developer.402pay.co/guides/accept-a-payment.md)[Fulfill orders reliably](https://developer.402pay.co/guides/order-fulfillment.md)

## Checkouts

A checkout is one attempt to pay a payment or link, in one rail, coin and network. A payment can have several, if the customer changes their mind, but only one can pay it: when the transfer lands on a quote the customer had left, the sibling they moved to closes with `replaced_by` naming the paid one, so open that one instead. IDs start with `chk_`.

[Hosted checkout](https://developer.402pay.co/guides/hosted-checkout.md)

## Remainders

When less than the amount due arrives, the payment becomes `underpaid`. A remainder is a checkout for exactly what's left, in the same coin and network, attached to the same payment. When the rest arrives, that payment succeeds, and no second payment is made.

[Request the rest](https://developer.402pay.co/guides/underpayments.md#request-the-rest)

## Payment links

A link is a reusable checkout with a fixed price. Every customer who pays it creates a new payment that carries the link's `link_id`. IDs start with `lnk_`.

[Payment links](https://developer.402pay.co/guides/payment-links.md)

## Customers

A customer is a person grouped by email, with their payment count and totals. One is added as soon as checkout collects an email, so a customer can have no payments yet. You can block a customer from paying you. IDs start with `cst_`.

[List customers](https://developer.402pay.co/api/customers/list.md)

## Events

An event records something that happened, such as `payment.succeeded`, with a snapshot of the object as it was. Webhooks deliver events, and you can list them any time. IDs start with `evt_`.

[Webhooks](https://developer.402pay.co/guides/webhooks.md)[Webhook event catalog](https://developer.402pay.co/guides/webhook-events.md)

## Wallet

Your wallet is self-custody: its recovery phrase is encrypted in your browser and never reaches 402pay. It keeps a balance per coin and network, and every payment's deposit is a wallet transaction. IDs start with `wal_` and `wtx_`.

[Wallet and deposits](https://developer.402pay.co/guides/settlement.md)[Reporting and reconciliation](https://developer.402pay.co/guides/reconciliation.md)