CheckOut 2 may initiate HTTP calls in certain situations to let the merchant know that a particular situation has happened. While it is possible (and perfectly OK) to implement a proper flow without using any server-side callback, they might be useful for more complex of non-traditional integrations.
Unlike the front-end callbacks, these have nothing to do with JavaScript or the customer’s browser. They represent HTTP calls made by CheckOut 2 back-end towards the merchant’s back-end, completely bypassing the customer’s browser.
