When a customer enters a coupon code at checkout, the system validates it through a multi-step process: Code Exists: Verify coupon with that code exists on server Date Check: Verify current date is between Starts At and Expires At (if set) Usage Check: Verify coupon hasn't exceeded max uses (if limit set) Client Check: If client restriction set, verify current user is that client Product Check: If product restriction set, verify coupon applies to at least one item in cart Status Check: Verify coupon is in valid/active state on server Valid: Coupon applied to cart, discount calculated, totals updated Invalid Code: Error message "Invalid coupon code" Expired: Error message "This coupon has expired" Not Started: Error message "This coupon is not yet active" Usage Limit Exceeded: Error