XRVA - Experimental RVA2 (class 3)
The normalize program writes these when creating a ID3v2.3 tag. It is the same as an RVA2 tag but has been back-ported to 2.3. There are various player plugins that understand this tag and adjust the volume appropriately.
Normalize is a tool for adjusting the volume of audio files to a standard level. This is useful for things like creating mixed CD's and mp3 collections, where different recording levels on different albums can cause the volume to vary greatly from song to song.