Each method has its own usages: PROCESS_EVENT is where you mainly handle events; notice that I'm using the word "mainly". GET_DATA is usually used to default/extract data once a user provides some values. GET_DEFINITION is used to pass the definition of the FPM UIBB to the calling application.
If you describe what you are trying to do, you might get a better answer to pinpoints where you should look.
By the way, if you want to enhance the behavior of the CR Header, you might want to review this document. It describes how you can add a custom field to the header. You don't have to do that of course but you can learn how to influence the header by creating a custom handler class (I just have a hunch that what you are looking for is the handler class).