The Validation API allows developers to provide their own validation of a cart and checkout, including express checkouts, such as PayPal, Shop Pay, Google Pay, and Apple Pay. > Note: > Errors from Validation functions are exposed to the Storefront API's [`Cart`](/docs/storefronts/headless/building-with-the-storefront-api/cart/manage) object, in themes using the [`cart` template](/docs/storefronts/themes/architecture/templates/cart) and during checkout. ## Example use cases - Require a customer membership at checkout. - Verify the age or ID of a customer when they proceed through checkout. - Provide B2B product minimums, maximums, and multiples. - Provide B2B location order minimums, maximums, or credit limits. - Specify quantity limits in a flash sale. ## Developer tools and resources
## Limitations and considerations - Validation functions are cached. This can result in a delay (typically, less than one minute) between changes being made and those changes being live. - In Point of Sale, Cart Checkout Validation Functions don't currently run, and operations aren't applied. - You can install a maximum of five validation functions on each store. ## Next steps - Learn how to use Validation functions by [validating a cart and checkout](/docs/apps/build/checkout/cart-checkout-validation/create-server-side-validation-function).