For software having a primary requirement to read or create RSS feeds.
RSS feeds are used to publish frequently updated information, such as blog entries, news articles, but also audio/video podcasts and more. The abbreviation stands for Rich Site Summary. Most often, an RSS document is written using xml, and includes full or summarized text, and metadata, like publishing date and author's name. Read more on Wikipedia.
Where should this tag be used?
Whenever reading or writing RSS feeds are a primary requirement to the software searched for, this tag is appropriate.