I literally just learned about this functionality yesterday — Medium calls them Text Shots. However I don’t think that’s relevant to Linda’s inquiry. If you drag to mark a section of someone’s article, you get a toolbar that pops up with four buttons. One of the buttons is the Twitter icon, which is what takes the snippet and turns it into a screenshot that you can tweet. That’s a Text Shot that you’re describing.

But the Highlighter button is a separate button in that toolbar. That’s what sends an item to the “Highlighted by” section of your profile. Just using your mouse to select text does not “Highlight” it from Medium’s point of view — you have to click the Highlighter button.

And nothing in this help file suggests that Text Shots will end up on your “Highlighted by” section. https://help.medium.com/hc/en-us/articles/214406358-Highlighting

So I’m pretty sure the people Linda is talking about are literally using the Highlighter button on their own stories, and I generally can’t find a good reason to do so.