A action (e.g. hire, terminate, re-hire, etc.) defines a follow up of UI screens, which must be filled up by the HR user to guarantee that all required information (personal, organizational, payment, etc.) are filled in for the employee record.
Different actions require different UI screens (information).
A reason is just a subspecification for an action. For a hire for instance you can specify, why an employee is hired: company expansion, just for fun, etc.
So for the SAP BL driver it might be, that you have combinations of action/reasons, which you are more or less interested in, so you have the choice to configure it in the GCVs.