xapG vs xmpG
From the XMP Specification (2005):
From the XMP Specification Part 2 (2008):
I Googled Adobe's site for clarification on this change, hoping to find a note on the subject: nada.
For the purposes of our XMP validator we're obviously going to assume that the most recent version is correct. The reason I made this blog post is so that it will pop up in Google when the next person stumbles into this question, wondering if it is a typo or a deliberate change.