In addition to connecting documents with metadata, two or more documents can be directly linked to each documents. The purpose is to allow navigating between two or more documents directly without having the navigate between them in the document list view. Applications for this might include indicating multiple documents that belong to the same original document, for instance different pages. Another example would be to link different versions of same document.
Links are always 1-1 between two documents. Multiple documents can be linked together, but each link will still be a 1-1 link. So, if we have four documents, A, B, C and D, and we link them:
The figure depicts the links between the documents. As a result, A is linked to B and B is linked to D. However, D is not linked to A.