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

# Key Identifiers

When integrating Tonder Hosted Checkout, you will encounter three different IDs. Understanding the purpose of each is essential for a smooth integration and for reconciliation.

### id (Session ID)

The Session ID is a unique string identifier for the Checkout Session itself. A session is the container for the entire payment experience, from the moment you create it to when it's completed or expires.

Example: `cs_97_41521_d11ba771527b4056c7f85786cfbb980bc105efaf42af113d`

You use the Session ID in the following ways:

* In the checkout URL Tonder provides, such as [https://stage-payflow.tonder.io/checkout/\{id}](https://stage-payflow.tonder.io/checkout/\{id}).
* When calling the [Get a Session](/hosted-checkout/api/get-session) endpoint to check the session's status.
* It's returned in the `return_url` or `success_url` query parameters.

### payment\_id (Payment Transaction ID)

The Payment Transaction ID is a unique numeric identifier assigned by Tonder for a single Payment Transaction. A transaction is a specific payment attempt, such as a card charge.

Example: `41521`

A single session might have multiple payment attempts if the first card is declined. For example:

* `41520` (Attempt 1: Declined)
* `41521` (Attempt 2: Success)

You can find the Payment ID in the following locations:

* In the Session object (`GET /sessions/{id}`) once a payment is attempted or successful.
* In webhook payloads, such as `session.completed` and `payment.transaction`.

You use the Payment ID when calling the [Get a Payment Transaction](/hosted-checkout/api/get-payment) endpoint to get details of a specific charge.

### external\_id (External ID)

The External ID is a user-defined string, such as `ORD-12345`. This is your internal identifier for the order, and you provide this ID when you create the payment session.

You use the External ID in the following ways:

* You send it in the [Create a Payment Session](/hosted-checkout/api/create-session) request.
* You use it to reconcile Tonder records with your own database.
* It's searchable in the [Tonder Dashboard](https://app.tonder.io).

### How They Relate

The diagram below shows how your system's order ID (`external_id`) maps to a Tonder Session, which can have multiple payment attempts:

```
Your System           Tonder's System
+-------------+       +-----------------------------+
| Order       |  1:1  | Session (id: string)        |
| (external_id)|------>| (external_id)               |
+-------------+       |                             |
                      |   1:N                       |
                      |   +----------------------+  |
                      |   | Payment (id: number) |  |
                      |   +----------------------+  |
                      |   | Payment (id: number) |  |
                      |   +----------------------+  |
                      +-----------------------------+
```

Here's a typical payment flow:

1. You create an Order, such as `ORD-500`, in your system.
2. You create a Session (e.g., `cs_97_41521_d11ba771...`) and pass `external_id: "ORD-500"`.
3. The user tries to pay and Tonder creates a Payment with `payment_id: 41520`.
4. The payment is declined.
5. The user tries again with a different card and Tonder creates a new Payment with `payment_id: 41521`.
6. This payment succeeds.
7. The Session is now marked `completed` and stores `payment_id: 41521` as the successful transaction.