In CATIA V5, a is a parent-child relationship. It tells one document (e.g., an Assembly) where to find another document (e.g., a Part).
If you meant a different kind of "link" (e.g., a from CATIA to an external document, or DMU link for kinematics), please clarify and I can provide a more targeted article. Alternatively, if you saw the phrase "CATIA V5 R33 link" in a forum or blog, please share the exact context or quote. catia v5 r33 link