When assigning courses to an audience, you are building the catalog of this audience at the same time. You can choose to assign courses to some audience by the Courses tab (1) when editing the details of the audience :
Once you setup the courses for that audience, only the users that belong to that audience will be able to browse their catalog and view those courses in order to enroll to them.
Notice!!!: If a course is assigned to some audience it will not appear either in the main catalog or in some branch catalog if it is also assigned to some branch