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

# Core Concepts

> The Billing data model and the three billing models

The Billing API is organised around three objects: **Customer**, **Account**, and **Bill**. These objects support three distinct billing models. A merchant typically operates with one billing model that reflects how their business tracks what customers owe.

***

## Billing Models

### Customer Balance

A balance is held directly on the customer and updated as payments come in. This is the simplest model and suits merchants who track a single running amount owed per customer without needing to break it down further.

The customer's `balance` field represents the current amount outstanding. A positive balance indicates an amount owed by the customer (a receivable). A negative balance indicates a credit held on behalf of the customer (a payable), most commonly in prepaid or wallet scenarios where the merchant holds value the customer has already funded. It also arises after an overpayment or credit adjustment. When a payment is received, Moment sends an `obligation.amount_applied` webhook and you reconcile the payment against the customer's balance.

👉 [Quickstart: Customer Balance](/api-reference/billing/quickstart-customer-balance)

### Account Balance

Balances are tracked across one or more accounts within a customer. Each account carries its own independent balance. This model suits merchants who need to separate what a customer owes across different products, services, or cost centres.

The customer record acts as a grouping entity. Balances live on the accounts below it. As with the Customer Balance model, an account's `balance` can be positive (amount owed) or negative (credit on the account). When a payment is received against an account, Moment sends an `obligation.amount_applied` webhook with the `account_id` of the account that was paid.

👉 [Quickstart: Account Balance](/api-reference/billing/quickstart-account-balance)

### Bill

Individual bills are raised against a customer or account with a specific `amount_due` and optional `due_date`. Each bill has its own lifecycle and payment status, tracked independently. This model suits merchants who issue discrete invoices.

When a payment is received against a bill, Moment sends an `obligation.amount_applied` webhook with the `bill_id` of the bill that was paid.

👉 [Quickstart: Bill Amount Due](/api-reference/billing/quickstart-bill-amount-due)

***

## The Data Model

### Customer

Customers are the top-level entity. Every billing relationship starts with a customer.

* A customer can carry a `balance` (used in the Customer Balance model). Positive values represent amounts owed; negative values represent credits.
* A customer can have multiple Accounts beneath it.
* A customer can have Bills raised directly against it.

### Account

Accounts are sub-entities within a customer used to track balances or bills at a finer level of granularity.

* An account belongs to exactly one customer.
* An account can carry a `balance` (used in the Account Balance model). Positive values represent amounts owed; negative values represent credits.
* An account can have Bills raised against it.

### Bill

A bill is a discrete payment obligation with a specific amount due.

* A bill must belong to either a customer or an account.
* A bill requires an `amount_due` and can optionally have a `bill_date` and `due_date`.
* A bill has a `status` of `unpaid` or `paid`.

### Relationships

* A **Customer** can have many **Accounts**.
* A **Customer** can have many **Bills** raised directly.
* An **Account** can have many **Bills**.