[Zope] Permissions: A question

Hi there, in CMF the rigth to add content is controlled by the permission "Add portal content" When I create my own portal type, say RR_content it gets automatically the "Add RR_contents" permission assigned . However I have the impression that to add portal content this permission is not taken into accout. It is still enough to have the permission "Add portal content" Do I miss something here ? What is that automatic permission for ?? thanks Robert