Purpose of PDF Annotation Tags???

I brought a highlighted and underlined PDF into a Scrivener project (see first image below).
When I clicked on the highlighted portions of the image, the red and yellow tags on the left appeared (see second image below – you might have to scroll or click on the image to see the tags).

I have been unable to determine the purpose of these tags. It seems that the red tag assumed the color of the red underline, and the yellow tag assumed the color of the yellow highlight. But I have been unable to write on either tag, or to move them. Clicking on the x in the upper right corner of each tag makes it disappear.

Can anyone explain the purpose of these tags? Thanks.

What you are looking at is the system PDF tool’s way of showing annotation notes for PDF. If you load the PDF in Preview, you should see similar note boxes in the columns. Why they are empty, I don’t know. When I drag an annotated PDF into Scrivener I can see the content of the note in the “sticky box” to the left. Perhaps these note boxes were left empty accidentally during the annotation process, or perhaps if you used something other than Preview to add them, they somehow got lost in the translation.

Thanks, Amber. The PDF in question was highlighted using Acrobat. Apparently, unlike Preview, Acrobat automatically adds a comment/note tag to each individual segment of highlighted text. Then, when the PDF highlight is clicked in Scrivener, an empty comment box shows up (empty, of course, unless a note/comment has been added while making the highlight in Acrobat – in which case that note/comment will be displayed when the highlighted text is clicked in Scrivener). Mystery solved!