Callbacks provide you with a mechanism to apply custom logic on different components of CPQ like line items, Cart page, Catalog page at run-time. For example, you can apply custom pricing or validation on the line items in the cart using Pricing Callback Class and Validation Callback Class. Callbacks are implemented using interfaces that are specific to each callback. These interfaces have various methods you can use to achieve your task. You must implement the interface in an Apex class and within that Apex class, you must configure your custom logic using the methods of the interface. Once you implement the interface, register that Apex class in Custom settings to invoke the callback class on run-time.