What is the vertical gray part of the icon in vim ui?

  question, vim

As shown in the figure, there are two split windows in the figure, and the one on the right appears when taglist is opened.
I specially added itset nuFor comparison.
What is the gray vertical bar on the leftmost side of the taglist window and how to control it?

taglist-split-window

That is calledsign column, you can look inside vim to help you understand it in detail.:help signs

To get rid of it, I think the simplest way is::sign unplace *, give it a try.