Ok so this is where I get confused and beg your indulgence ?
So it's in the photoshop namespace and it's not going to be an embedded value but the PLUS standards are now part of the latest published 1.4 standard - are they not XMP only fields as well ?
Who curates / maintains / publishes IPTC4XMP if not IPTC ?
The Metadata Standards Working Group (MWG) and the International Press Telecommunications Council (IPTC) but when I say IPTC vs XMP, I am talking about the obsolete binary IPTC format versus the XML-based XMP format.
Perhaps if you rephrase your question I can try again to answer. The Copyright URL field in PM is represented in XMP as "WebStatement" in the XMP Rights namespace "
http://ns.adobe.com/xap/1.0/rights/".
So basically when you enter a Copyright URL and then save the metadata, it is written to XMP like so:
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMP Core 5.1.2">
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<rdf:Description rdf:about=""
xmlns:xmpRights="http://ns.adobe.com/xap/1.0/rights/"
xmpRights:WebStatement="http://www.someurl.com/"
</rdf:Description>
</rdf:RDF>
</x:xmpmeta>
Specifically like this: xmpRights:WebStatement="
http://www.someurl.com/"
When PM loads XMP metadata, it looks for the WebStatement and puts it in the "Copyright URL" field.
I hope this makes things clearer.
-Kirk