Form 'code_base': Here are all the functions like 'new record', 'delete record' ect. On this form there are no parts, it is strictly code. This form will have no datasource.
Form 'data_controller': Here are buttons for the actions 'new record' and 'delete record' ect. There could be more than one, so there can be multiple designs. This form will inherit the 'code_base' form and uses all it's functions. This form will have no datasource.
Form 'customers': This will inherit the 'data_controller' because the functions and buttons are already inherited only the datasource and fields have to be added and there is a working form.
Setting a super form
On an existing form, the super form can be set by going to the properties and setting the "extends" property to the super form.