Software that reads or modifies XML content.
XML - eXtensible Markup Language - is an open standard for describing data in a simple, highly flexible, human-readable text format, which has been specifically designed to store and transport data.
XML is mentioned in RFC 3023, which lays out the rules for the construction of internet media types for use when sending XML. RFC 3023 defines media types
text/xml, which merely define that data is in XML format, not stating anything about its semantics.
Where should this tag be used?
Whenever reading or writing XML is a primary requirement, this is your tag. This also includes formatting, syntax highlighting/verifying, and editing.