When assigning courses to an audience, you are building the catalog of this audience at the same time.
Note: When an audience is created in a branch by a Branch Administrator, then only the users of that specific and its sub-branches can be added. Even if this audience is edited and modified by a Main Administrators afterwards, still users will be filtered according to the branch where the audience was created.
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 enrol to them.
Note: 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.