> ## Documentation Index
> Fetch the complete documentation index at: https://nayax-44d6e37b-docs-marshall-faq-redistribution.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# API Introduction

The Marshall Pro (Fuel) protocol is a high-performance, command-based interface designed for fuel retail environments. It allows your Point-of-Sale (POS) or Forecourt Controller (FCC) to manage the full lifecycle of a transaction—from card detection to final settlement.

Unlike cloud APIs, the Marshall Pro (Fuel) protocol runs locally on your station's network. The VPOS terminal acts as a persistent TCP/IP server waiting for your application to connect.

The API is categorized into four functional areas to help you build a complete customer journey:

<CardGroup cols={2}>
  <Card title="Payment Processing" icon="credit-card">
    Supports single-phase (`pay`) and two-phase (`authorize` + `settle`) fuel transactions.
  </Card>

  <Card title="Card Detection" icon="magnifying-glass">
    Read card data without an immediate charge—perfect for Loyalty and Customer ID.
  </Card>

  <Card title="User Interface" icon="desktop">
    Take control of the terminal screen to show custom menus, fuel types, or prompts.
  </Card>

  <Card title="Peripheral Support" icon="print">
    Directly trigger the integrated thermal printer for customer receipts.
  </Card>
</CardGroup>

See [Request and Response Format](/docs/integrate-pos-device/marshall-pro/messaging-protocol/request-response-format) for more information.

<Note>
  **Socket Management:** The VPOS server maintains a single active session. If your application opens a new socket, the server automatically terminates the previous connection to ensure data integrity.
</Note>