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

# Main domains

> Explore Pix's four core domains — DICT keys, transactions, QR Codes, and MED refunds — plus BACEN principles for interoperability, security, and settlement.

Each domain owns a specific part of the instant payment flow. Together, the domains provide interoperability, security, and real-time execution across the ecosystem.

This overview introduces the four core domains that form the foundation of Pix.

Each child page in this section gives more detail.

## Core Pix domains

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### DICT — Pix Key Directory

The DICT is the national registry of Pix keys.

It maps simple identifiers (CPF/CNPJ, phone, email, random key) to transactional accounts. Users send and receive Pix without full bank details.

### Pix Transactions

This domain manages the execution lifecycle of an instant payment — authorization, validation, settlement, status updates, and traceability.

Every transaction flows through regulated paths and must settle within seconds in the SPI infrastructure.

### QR Codes

Pix supports static and dynamic QR Codes for payment initiation.

Static codes are reusable. Pix generates a dynamic code for each transaction. The dynamic code can carry the amount, metadata, and expiration.

### MED — Special Refund Mechanism

The *Mecanismo Especial de Devolução* governs fraud-related refund scenarios.

It defines mandatory rules, timelines, and workflows for disputes, error handling, and fraud investigation across institutions.

## Additional Pix characteristics

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Pix also follows system-wide principles for safety, inclusiveness, and nationwide compatibility.

### Universal interoperability

Every participant in Pix communicates through shared standards — no closed networks, no proprietary protocols.

Any participant can send and receive Pix with any other.

### Regulatory standardization

BACEN defines the rules, message formats, limits, fraud controls, and operational requirements.

All participants must follow the same specifications.

### Open participation model

Pix supports many types of institution:

* Banks
* Payment institutions
* Fintechs
* Digital wallets
* Cooperatives
* Government entities

### Security posture

Pix enforces a strict security framework with:

* Strong encryption
* Standardized authentication and authorization
* Mandatory fraud-prevention processes
* Full auditability and traceability of every event