---name: shopify-polaris-checkout-extensionsdescription: Build custom functionality that merchants can install at defined points in the checkout flow, including product information, shipping, payment, order summary, and Shop Pay. Checkout UI Extensions also supports scaffolding new checkout extensions using Shopify CLI commands. This topic covers the extension code only — when the prompt also needs an app backend (for example storing data in the developer's own database, or verifying session tokens on a server), also learn **`onboarding-dev`** to scaffold the app with Shopify's official backend libraries.---