Does anyone know what the "link" toggle button in the upper right hand of the table of contents view (LCD side) does? The other buttons in that row are meant to show/hide notes, highlights, bookmarks, annotations, attachments and TOC entries. I thought this might be related to URL links, but tried to test that theory and don't think that's it. The user manual doesn't explain it either (or really describe the table of contents).

Does anyone know what this is for?