# Core Entities

epilot ships with a set of built-in entity schemas that model common business objects. This page lists all core schemas with their descriptions, relationship diagram, and links to API docs.

See the [API Changelog](/api/changelog) for recent changes to entities and APIs.

## Relationship Diagram

```mermaid
graph TB
    %% Customer Relations
    Account[Account]
    Contact[Contact]

    %% Sales
    Opportunity[Opportunity]
    Order[Order]

    %% Products
    Product[Product]
    Price[Price]
    Tax[Tax]

    %% Contracts & Billing
    Contract[Contract]
    BillingAccount[Billing Account]
    BillingEvent[Billing Event]

    %% Metering
    Meter[Meter]
    MeterCounter[Meter Counter]

    %% Sales Flow
    Opportunity -->|has many| Order

    %% Customer Relations
    Contact -->|has many| Opportunity
    Contact -->|has many| Order
    Contact -->|has one| Account
    Contact -->|has many| Contract
    Contact -->|has many| Ticket
    Contact -->|has many| BillingAccount

    Order -->|has many| Product

    %% Products
    Product -->|has many| Price
    Price -->|has one| Tax

    %% Contracts & Billing
    Contract -->|has one| BillingAccount
    Contract -->|has one| Order
    Contract -->|has many| Meter

    BillingAccount -->|has many| BillingEvent

    %% Metering
    Meter -->|has many| MeterCounter

    style Account fill:#e1f5ff
    style Contact fill:#e1f5ff

    style Opportunity fill:#fff4e1
    style Order fill:#fff4e1

    style Product fill:#f0e1ff
    style Price fill:#f0e1ff
    style Tax fill:#f0e1ff

    style Contract fill:#e1ffe1
    style BillingAccount fill:#e1ffe1
    style BillingEvent fill:#e1ffe1
```

## Built-in Entity Schemas

### Customer Relations

### Sales

### Product Hub

### Contracts & Billing

### Metering

### Journeys

### Messaging

### Marketing & Analytics

### System