In Magento Open Source, when would a Zero Subtotal checkout be used?

The Zero Subtotal Checkout payment method is specifically designed for situations where the customer's grand total is exactly zero. This scenario occurs when the total cost of all items in the cart, including any applicable shipping fees and taxes, is completely covered by a discount, coupon code, gift card, or store credit. When the balance due is zero, this method allows the customer to complete the checkout process without entering traditional payment information, as no payment is required.

A. If the discount does not cover the full value of the products, the grand total will be greater than zero, requiring a standard payment method.
B. If a shipping charge remains after the discount, the grand total is not zero, and the customer must use a different payment method to pay for shipping.

1. Adobe Commerce Documentation, Payment Methods, Zero Subtotal Checkout: "The Zero Subtotal Checkout payment method is available only when the grand total of an order is zero. An order with a zero grand total can occur when a discount is applied that is equal to the grand total... When the grand total of an order is zero, the Zero Subtotal Checkout method becomes the only payment method available during checkout." (Adobe Experience League, User Guide, "Payment Methods", Section: "Zero Subtotal Checkout").