To implement the Apex method that is executed when a recommendation is rejected, the developer should keep in mind the following factors:
* The method must use the @Future annotation because it performs a callout to an external system, which is not allowed in synchronous Apex. The @Future annotation marks a method as asynchronous, meaning that it runs in a separate thread and does not block the main execution1.
* The method must use the @invocableMethod annotation because it is invoked from a flow that is triggered by the Einstein Next Best Action component. The @invocableMethod annotation enables an Apex method to be called from a flow, process, or REST API2.
* The method must be defined as static because it is annotated with @Future and
@invocableMethod. Both annotations require the method to be static, meaning that it belongs to the class and not to any instance of the class3.
References:
* 1: Asynchronous Apex
* 2: Invocable Apex
* 3: Static and Instance Methods, Variables, and Initialization Code