It's a link to a specific post within a thread, not a topic.
This link is a link to this specific post, rather than the top of the topic. (it's more obvious (and useful)for longer threads)
Note I needed to edit this post to get the link, as there is no way for me to select Permalink while composing the response.
I viewed my post, hit Permalink, it jumped so my post was at the top of the browser, I then copied and pasted the address (using the link icon) to set the Link.
-Dave