In the "Edit" mode of a course or curriculum, Learning Authors will see the option to select either an "Open" or a "Limited" enrollment type.
Open enrollment means that users can self-enroll in the content. It will show in the Course Catalog for anyone with viewing permissions to the folder the content is in.
Limited enrollment means that users need to be enrolled in the content in order to see it, either individually or through a group enrollment. It will not show in the Course Catalog to allow students to self-enroll.