> ## Documentation Index
> Fetch the complete documentation index at: https://docs.y.uno/llms.txt
> Use this file to discover all available pages before exploring further.
# Understanding Flows
In Yuno SDKs, **payment flows** and **enrollment flows** are separate. Payment is the process of completing a transaction; enrollment is the process of saving a payment method (e.g. a card) to a customer account for future use. This page explains both flows at a high level with diagrams and step sequences. Use the Quickstart guide and platform flow pages for implementation details.
## Payment flow
The SDK provides a unified payment experience: customers complete transactions using multiple payment methods within a single integration. The diagram below illustrates the main payment workflow.
RequestResponseSuccess
### Payment flow steps
1. Merchant Server: `Create Customer` → Yuno Server: `Creates Customer`
2. Merchant Client: `Initiate Checkout` → Merchant Server: `Create Checkout session`
3. Merchant Server: `Create Checkout session` → Yuno Server: `Creates Checkout session`
4. Merchant Client: `Initiate Checkout` → `Initiate SDK` to continue payment flow
5. Merchant Client: `Initiate SDK` to continue payment flow → Yuno SDK: `Receive checkout session`
6. Yuno SDK: `Receive checkout session` → `List payment methods`
7. Yuno SDK: `List payment methods` → User selects payment methods
8. Yuno SDK: User selects payment methods → Callback with the `one-time token`
9. Yuno SDK: Callback with the `one-time token` → Merchant Client: Receive `one-time token` (single use)
10. Merchant Client: Receive `one-time token` (single use) → `Initiate payment`
11. Merchant Client: `Initiate payment` → `Initiate SDK` to continue payment flow
12. Merchant Client: `Initiate SDK` to continue payment flow → Yuno SDK: `Continue with the payment flow`
13. Yuno SDK: `Continue with the payment flow` → Show screen for the user to complete payment
14. Yuno SDK: Show screen for the user to complete payment → Display `payment result` (optional)
15. Merchant Client: `Initiate payment` → Merchant Server: `Create payment`
16. Merchant Server: `Create payment` → Yuno Server: `Creates payment` in the payment provider
### Payment flow using a vaulted token
If a customer has an enrolled payment method, they can use a **vaulted token** to pay without entering payment details again.
RequestResponseSuccess
#### Vaulted token flow steps
1. Merchant Client: `Initiate checkout` → Merchant Server: `Create Customer`
2. Merchant Server: `Create Customer` → Yuno Server: `Create Customer`
3. Merchant Client: `Initiate SDK` with the `checkout session` → Yuno SDK: `Receive checkout session`
4. Yuno SDK: `Receive checkout session` → `List payment methods`
5. Yuno SDK: `List payment methods` → User selects payment methods
6. Yuno SDK: User selects payment method → Callback with the `one-time token`
7. Yuno SDK: Callback with the `one-time token` → Merchant Client: `Initiate payment`
8. Merchant Client: `Initiate payment` → Merchant Server: `Create payment`
9. Merchant Server: `Create payment` → Yuno Server: `Creates payment` in the payment provider
10. Merchant Client: Show `payment result` → Merchant Server: `Receive payment result`
11. Merchant Server: `Receive payment result` → Yuno Server: Provides `payment result`
## Enrollment flow
**Enrollment** is the process of saving a payment method (e.g. a card) to a customer’s account so it can be used later (e.g. with a vaulted token). full-checkout supports enrollment; the diagram below describes the enrollment workflow.
RequestResponseSuccess
### Enrollment flow steps
1. Merchant Server: `Create Customer` → Yuno Server: `Creates Customer`
2. Merchant Client: `Add payment method` → Merchant Server: `Create customer session`
3. Merchant Server: `Create customer session` → Yuno Server: `Creates customer session`
4. Merchant Client: `Display payment methods to enroll` → Merchant Server: `Request available payment methods to enroll`
5. Merchant Server: `Request available payment methods to enroll` → Yuno Server: `Returns available payment method`
6. Merchant Client: `Display payment methods to enroll` → Merchant Client: User selects payment method to enroll
7. Merchant Client: User selects payment method to enroll → Merchant Client: `Initiate enrollment`
8. Merchant Client: `Initiate enrollment` → Merchant Client: `Initiate SDK` to continue enrollment
9. Merchant Client: `Initiate SDK` to continue enrollment → Yuno SDK: `Continue enrollment flow`
10. Yuno SDK: `Continue enrollment flow` → Yuno SDK: Shows screens for the user to complete enrollment
11. Merchant Server: Receive `enrollment result` via webhook → Yuno Server: Receive `enrollment results` from payment provider
12. Yuno Server: Receive `enrollment results` from payment provider → Yuno SDK: Display `enrollment result` (optional)
## Headless SDK flow
The **Headless SDK** provides maximum flexibility by allowing you to manage the entire user interface while Yuno handles the complex payment logic and security (like 3DS).
RequestResponseSuccess
### Headless flow steps
1. **Merchant Server**: `Create Customer` → **Yuno Server**: `Creates Customer`
2. **Merchant Client**: `Initiate Checkout` → **Merchant Server**: `Create Checkout session`
3. **Merchant Client**: `Collect Information` → User enters card/payment details in your custom UI.
4. **Merchant Client**: `Generate Token` → **Yuno SDK**: Call `generateToken(details)` to receive a `one-time token`.
5. **Merchant Client**: `Process Payment` → **Merchant Server**: Send `one-time token` to your backend.
6. **Merchant Server**: `Create Payment` → **Yuno Server**: Call [Create Payment](/reference/payments/create-payment) API.
7. **Merchant Client**: `Handle Actions` → If API response returns `sdk_action_required: true`, use the SDK to handle 3DS or other [Payment Actions](/docs/sdks/headless-web/payment#step-7-get-payment-actions).