ATOM feeds (RFC 4287) are an alternative representation of content flowing on a page — typically reverse-chronological posts of some sort — like blog posts. Autodiscovery of these feeds is made possible through a <link> tag in the head of the HTML document:

<link rel="alternate" type="application/atom+xml" href="http://www.example.com/xml/index.atom" />

To aid in discovery of activity streams, a couple options have been presented:

1. <link rel="activitystream" type="application/atom+xml" href="http://www.example.com/xml/index.atom" /> or <link rel="alternate activitystream" type="application/atom+xml" href="http://www.example.com/xml/index.atom" />
2. <link rel="alternate" type="application/atom+xml" href="http://www.example.com/xml/index.atom" class="activitystream" />

Though it is possible to point to a stream for comments on a given blog post, let's say, there's no way to indicate that it's actually a comment feed (except in the human-friendly title):

<link rel="alternate" type="application/atom+xml" href="http://www.example.com/xml/comments.atom" title="Recent comments feed" />

It may not be necessary to call out the activitystream type of feed, but seems useful to make it easier to provide a hint to aggregators that a particular feed can be consumed as an activity stream.

This was also discussed on the mailing list.