No idea, you'd have to ask the Discourse folks that (meta.discourse.org).
Since I process the forum mostly in Mailing List Mode in Mail, I can just control click on the Reply links to get the link so I rarely see it myself.
The "slug" (for this topic for example
why-the-user-name-parameter-in-links-to-replies is actually irrelevent, you can put anything you want there.
The t at the start says it is a topic, and the number is the topic ID(
5706 in this case). After that you can indicate a specific reply by adding that to the end (eg
I'm afraid I cannot parse that, so I don't know how to answer it, sorry. Perhaps you can rephrase it?