Aggregators may have limits on the URI schemes they support.
Content developers should not assume that all aggregators support all schemes.
The data in these elements must begin with an IANA-registered URI scheme, such as https://, news://, mailto: and ftp://.
Prior to RSS 2.0, the specification only allowed and ftp://, however, in practice other URI schemes were in use by content developers and supported by aggregators.
It's used in the post command in the Radio User Land aggregator.
It should be generated automatically when forwarding an item from an aggregator to a weblog authoring tool. It has one optional attribute, domain, a string that identifies a categorization taxonomy.
The value of the element is a forward-slash-separated string that identifies a hierarchic location in the indicated taxonomy.