Adina Levin has suggested that we clarify how to apply tags to activities. This is a good idea, and fortunately one that doesn't require a modification of the spec or format, but rather a clarification of how one tags items in ATOM feeds.
Use cases include:
Syndicating or aggregating by topic. Let’s say I have a stream of messages talking about web standards, politics, and music. I want to syndicate only music-related updates to a music site. I use a tag that identifies a given update as a music-related update. Then, I can choose to share only those updates to the music site.
Syndicating or aggregating by person. The practice is to labelling things with people’s names so they can find themselves.With flickr the practice is tacit (ie names are general tags). Delicious has the for:kevinmarks convention, and in FaceBook, this practice is just called tagging, as it is the primary mode of use.
Specifically, you can use the <category> element of ATOM to express categories, labels, or tags:
<category scheme="http://myblog.com" term="Cats" /> <category scheme="http://myblog.com" term="Dogs" />
However, using adjusting the scheme attribute, you can also differentiate between categories and tags, as one might on a WordPress blog:
<category scheme="http://myblog.com/category/" term="cats" label="Cats" /> <category scheme="http://myblog.com/category/" term="dogs" label="Dogs" /> <category scheme="http://myblog.com/tag/" term="dogs" label="Dogs" />