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

# Core Payment Sequences

> The two primary interaction patterns for initiating payments in ACK-Pay.

ACK-Pay supports two primary interaction patterns for initiating payments, both centered around a standard [Payment Request](/ack-pay/payment-request-payload) payload:

1. **Server-Initiated Sequence:** A Server (or Service Provider) requires payment from a Client agent before granting access to a resource or completing an action. In this flow, the **Server generates and sends** the Payment Request to the Client, typically in response to an initial resource request. This is common for pay-per-use APIs or accessing paid content.

   [Learn more about the Server-Initiated Sequence →](/ack-pay/server-initiated-sequence)

2. **Client-Initiated Sequence:** A Client agent initiates a payment based on an external trigger or known obligation (e.g., paying an invoice, completing an e-commerce checkout, fulfilling a human end-user’s instructions). In this flow, the **Client agent constructs** the Payment Request itself, often by parsing external information (like an invoice or checkout summary) and potentially using a Payment Service to help formulate the request details before interacting with the Payment Service API for execution.

   [Learn more about the Client-Initiated Sequence →](/ack-pay/client-initiated-sequence)

Both sequences leverage the same core [Components and Roles](/ack-pay/components-roles) (Client, Server, Payment Service, Receipt Service, Settlement Networks) and the [ACK Receipt](/ack-pay/receipt-verification) for verification.