The IMG (Implementation Guide) in SAP provides you with the various configuration steps in a tree-like structure for easy access with the nodes at the bottom representing the configuration objects. This is the central repository for customizing, providing a step-by-step guide for carrying out various activities. Besides the steps/activities, the IMG also contains explanations concerning the order in which you need to make the customizations. When you execute an activity from the IMG, you are indirectly changing the values (parameters) in the underlying table.
The SAP Reference IMG provides all the customizing steps for all functional areas of SAP. This, as the name suggests, is the reference IMG from which you may create your own IMG to meet the exact requirements of the enterprise and project.
The Enterprise IMG is usually an exact copy of the SAP Reference IMG, but limited to the countries where the implementation is carried out. From the Enterprise IMG, you may create your Project IMG, which will contain the application components/business processes required in the current project. It is also possible to create the Project IMG by directly generating it from the SAP Reference IMG.
In this case, the country selection is done when the Project IMGs are created.