The UIM LINK element allows links to other client pages to be specified indirectly. The PAGE_ID_REF attribute is a key into the CuramLinks.properties file that returns the actual ID of the linked page.
Many links can point to the same page reference. The advantage of using a page reference is that all the links can be updated by changing a single entry in this file.
Each component can have its own CuramLinks.properties file. During generation, these individual files will be merged. As usual, if a particular key is present in more than one CuramLinks.properties file, the component priority order is used to decide which value is retained.