Purpose of +xml in HTTP MIME type

Question

What's the significance of the +xml in the following HTTP Accept Header:

application/vnd.google-earth.kml+xml

Is that just to denote it's an XML based format, or that it's suitable for XML editors, or something completely different?

http://www.iana.org/assignments/media-types/application/vnd.google-earth.kml+xml

Answer 1

Check the RFC :

Appendix A. Why Use the '+xml' Suffix for XML-Based MIME Types?

Although the use of a suffix was not considered as part of the original MIME architecture, this choice is considered to provide the most functionality with the least potential for interoperability problems or lack of future extensibility. The alternatives to the '+xml' suffix and the reason for its selection are described below.

There is a whole list of reasons listed underneath, but I don't think copying them verbatim falls under fair-use. So check the RFC for their full (long!) story of how this came to be.