Link button in table of contents view?

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?

looking back at old posts, noticed this one dropped off without any comments. Anyone have any guesses?

I've been wondering this myself. Perhaps it toggles the link between the file that stores the notes, bookmarks, etc. and the pdf/epub file. I'm not sure why this would be useful though. I suppose if one of the notes became corrupt and kept the pdf from opening you could turn the link off and see if the pdf would open??

No idea either, devseev. The best I can do is this: When I first open the TOC of a book with bookmarks, notes, etc., the book icon at the end of the row is off (grayed). I tap the link icon (turn to gray), then tap the book icon and the link icon again. It seems a roundabout way to list the annotations, but I'm sure there's more to it and I just can't figure it out.

For a while, every time I'd read a book I'd tap the link icon: on, off, on, off. Nothing. Finally, I stopped when I remembered the old Steven Wright joke: "I have a switch in my house that does nothing. For the heck of it, every time I walk in the room, I flick it on and off a few times. Last week I got a call from a woman in Germany. She said, 'Knock it off.'"

(Just saw Boris' post. Yes, lock/unlock it is. And now, for Boris' next trick, he'll tell us what it does. But no pressure....)