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

# Glossary

> The vocabulary of agentic payments — x402, MPP, HTTP 402, Circle Gateway, the Transactability Score, self-custody, and the SELAT components — defined in one place.

The terms an agent developer meets when paying for APIs at runtime. Each links to the primer or reference that covers it in depth.

### 402 challenge

The payment instruction a server returns with an `HTTP 402` response: what to pay, in what asset, to whom, on which network. The client pays and retries with proof. See [HTTP 402](/docs/primers/http-402).

### Agentic payments

Autonomous agents paying for services themselves — discovering a capability mid-task, paying per call, and continuing — without a human provisioning accounts or keys first. See [Agentic payments](/docs/primers/agentic-payments).

### Circle Agent Wallet

An MPC self-custody wallet for an agent, run through Circle. The signing material stays with the owner; SELAT never holds keys or funds. It is the wallet SELAT pays from.

### Circle Gateway

Circle's mechanism for holding a unified USDC balance across chains and moving it instantly via deposit, burn, and mint. SELAT funds through it so an agent can pay on whatever network a merchant quotes. See [Nanopayments powered by Circle Gateway](/docs/primers/nanopayments-gateway).

### ERC-3009 (EIP-3009)

The `transferWithAuthorization` standard (ERC-3009, also referenced as EIP-3009) that lets a USDC transfer be authorized by an off-chain signature — the mechanism behind an x402 `exact` payment on EVM. See [x402](/docs/primers/x402).

### Federated catalog

SELAT's merged, deduplicated index of paid services across source registries (Circle's x402 catalog, MPP, pay.sh, Apify, and more). Discovery ranks over it. Searchable free via the [Catalog MCP](/docs/selat-mcp) or the [SELAT CLI](/docs/selat-cli).

### HTTP 402

The `402 Payment Required` status code — dormant for decades, now the foundation both x402 and MPP build on. See [HTTP 402](/docs/primers/http-402).

### MPP (Machine Payments Protocol)

A multi-rail payment protocol co-authored by Stripe and Tempo, built on `HTTP 402`. It settles stablecoins, cards, and custom methods. See [MPP on Tempo](/docs/primers/mpp-tempo) and [x402 vs MPP](/docs/primers/x402-vs-mpp).

### Rail

A payment protocol-and-network an endpoint accepts — for example x402 (USDC on EVM/Solana) or MPP (Tempo, Stripe). The merchant chooses the rail; SELAT translates one signed shape into whichever the upstream speaks.

### Self-custody

The user holds the keys and the funds; no intermediary can move them. SELAT is self-custody by design — it discovers, prices, and routes, but never holds keys or balance.

### Tempo

The chain MPP settles its native stablecoin payments on. One of MPP's production methods — not the only one. See [MPP on Tempo](/docs/primers/mpp-tempo).

### Transactability Score

SELAT's per-endpoint reliability signal, delivered on the `402` challenge: what has actually settled (success rate, paid count, last status) for an endpoint. The aggregate system is the **Transactability Index**. See [Transactability Index](/docs/transactability).

### USDC

The dollar stablecoin x402 settles in (and one of MPP's methods). Payments in SELAT settle in USDC from the user's Circle Agent Wallet.

### x402

An open `HTTP 402` payment protocol from Coinbase, now under the Linux Foundation. USDC-native, on-chain, pay-per-call. See [x402](/docs/primers/x402) and [x402 vs MPP](/docs/primers/x402-vs-mpp).