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 - Use [tokengating](/docs/apps/build/blockchain/tokengating) or 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 <div class="resource-card-grid"> <div> <a class="resource-card" href="/docs/apps/build/cli-for-apps" data-theme-mode=""> <div class="resource-card__indicator-container"><img src="/assets/resource-cards/cli" data-alt-src="/assets/resource-cards/cli-dark" aria-hidden="true" class="resource-card__icon themed-image"></div> <h3 class="resource-card__title"> Shopify CLI </h3> <p class="resource-card__description">A command-line tool for building Shopify apps.</p> </a> </div></p> <p><div> <a class="resource-card" href="/docs/api/functions/reference/cart-checkout-validation/graphql" data-theme-mode=""> <div class="resource-card__indicator-container"><img src="/assets/resource-cards/graphql" data-alt-src="/assets/resource-cards/graphql-dark" aria-hidden="true" class="resource-card__icon themed-image"></div> <h3 class="resource-card__title"> Validation API reference </h3> <p class="resource-card__description">Consult the GraphQL reference for the Validation API.</p> </a> </div> </div> ## 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. - 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).