> ## Documentation Index
> Fetch the complete documentation index at: https://docs.useroutr.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Core model

> Rails, endpoints, and the two funding primitives. The vocabulary every other page assumes.

Four ideas carry the whole API. Learning them takes about five minutes and makes everything else predictable.

## A rail is a rail

A rail is how value moves. A blockchain and a bank network are the same kind of object: both have an address format, a settlement time, a fee model, and a set of failure modes.

```json theme={null}
{
  "id": "stellar",
  "kind": "chain",
  "directions": ["in", "out"],
  "requires_memo": true,
  "finality": { "typical_seconds": 5, "p95_seconds": 8 },
  "assets": ["crypto:USDC", "crypto:XLM"]
}
```

`GET /rails` returns the live set. Rail identifiers are open strings, not a fixed enum, and new rails appear without an SDK release. Any `switch` you write over a rail needs a `default`.

<Note>
  Some rails support only one direction. Check `directions` before using a rail as a destination.
</Note>

## An endpoint describes both ends

One shape covers a blockchain address, a bank account, a card, and an internal balance. Sources and destinations use the same type: direction is a property of the transfer, not of the type.

<CodeGroup>
  ```json Crypto theme={null}
  {
    "rail": "stellar",
    "asset": "crypto:USDC",
    "target": { "kind": "chain_address", "address": "G...", "memo": "12345" }
  }
  ```

  ```json Bank theme={null}
  {
    "rail": "ach",
    "asset": "fiat:USD",
    "target": { "kind": "bank_account", "external_account_id": "ext_..." }
  }
  ```

  ```json Balance theme={null}
  {
    "rail": "ledger",
    "asset": "credit:acme:usd",
    "target": { "kind": "ledger_account", "account_id": "la_..." }
  }
  ```
</CodeGroup>

The practical consequence: once you can accept money, you can send it. Paying out is the same call with the endpoints the other way round.

## Assets are namespaced

`crypto:USDC`, `fiat:NGN`, `credit:acme:ai_tokens`. An asset is a symbol at the model layer, resolved to a contract address, issuer, or mint by the registry per rail. That is what lets one intent quote across eight chains without eight asset identifiers.

<Warning>
  The same symbol has a different scale on different rails. USDC is 6 decimals on EVM and 7 on Stellar. Never hardcode a scale: take it from the registry.
</Warning>

Amounts are always decimal strings, always paired with an asset. Never a float, never a bare number.

## Two funding primitives

<CardGroup cols={2}>
  <Card title="Funding intent" icon="receipt">
    One funding event. Has an amount or is open, expires, and terminates.
    Use for checkout and anything invoice shaped.
  </Card>

  <Card title="Funding address" icon="inbox">
    A persistent, reusable receive endpoint bound to a destination.
    Any deposit creates an intent automatically. Use for user deposit addresses.
  </Card>
</CardGroup>

They share one settlement path, so everything downstream behaves identically.

## Legs and routes

A route is an ordered list of legs. Each leg has a `kind` describing what it does, separate from the provider performing it.

| Kind       | What it does                                                         |
| ---------- | -------------------------------------------------------------------- |
| `transfer` | Move on one rail, no conversion                                      |
| `swap`     | Convert asset, same rail                                             |
| `bridge`   | Move between chain rails                                             |
| `ramp`     | Cross the fiat and crypto boundary, either direction                 |
| `credit`   | Write the ledger credit. Always the final leg when funding a balance |

You never choose a provider. You can express preference through `policy`, and if we cannot satisfy it we return an error rather than silently ignoring it.