posthumanzzz
if you click "reply" it puts the post you are replying to in a format which quotes it.
that triggers an alert to the person who you are quoting, saying that you've replied to them.
i know for a certainty you have received a great number of these very such alerts, and i expect you've been appreciative of the service the notification provides.

you can also highlight text in anyone's post, and an button appears that says "reply" next to the highlighted text. if you click it, it automatically formats a quote of exactly what you have highlighted in the reply box.

when you use quotes properly, it benefits everyone in the forum. the person you reply to knows you are talking to them, without randomly stumbling on your post, and everyone who reads it can click on the quote box in your post and be directed to the original post you quoted, so they can see for themselves what the other poster has said, in full context. that's useful because, as i'm sure you are aware, sometimes humans misrepresent facts.
you're welcome
