Why no elements in my bookmark setup? What is Thread by Colum?

I use structured FM with tags and formats defined in and inherited from my old company. When I print book, select PDF setup > Bookmarks, select Elements in Bookmark Source, the Dont's include and Include Paragraphs column are all empty.

P.S. I read the guide of FM, but still cannot understand what is the use of Thread by Colum and Thread by Text Frame.

Thanks in advance.