How does hn decide what articles to put on rss?

They're in the same order as the front page

If the /rss query always return the current front page order articles, then I wonder how RSS subscription services work to mark some of them read and some not.

Should probably need to know how RSS actually works.

Well formed feeds have unique id's per item, that stay the same even with title or order or timestamp changing.

I think it's actually /newest. I don't remember the RSS feed ever rearranging stories up and down the list.