To increase the interoperability and reusability of learning objects, Advanced Distributed Learning Initiative developed a model called Content Aggregation Model (CAM) to describe learning objects and express relationships between learning objects. However, the suggested relations defined in the CAM can only describe structure-oriented relationships and cannot express semantic relationships between learning objects. Although extended relations were proposed in the past, some of the proposed relations are redundant and even inappropriate. In addition, the usefulness of these relations has never been formally studied. To solve the problems, we systematically studied these relations from authors' perspective and proposed an extension to CAM. The extension was tested by 30 authors using a web-based learning content management system that was developed by us. (Contains 7 tables and 8 figures.)