What's this called (hashtag column) and what's the advantage. And how do I turn it OFF?

Screenshot 2023-10-24 095038.png

E2#

When I'm selecting a column via mouse, I sometimes get this. Why?

That's the first question.

The second question is when I'm selecting a column via mouse, I sometimes don't get this. Why?

Third, what's the point of this?

And finally, how do I turn it off?

Thanks.

PS: I tried Googling it. I dont even know what this thing is called. And Googling "hashtag" just gives a bunch of results about hashtags in cells that are too skinny. That's not what this is about.