Extensible Metadata Platform
https://pdfa.org/resource/iso-16684-xmp/
ISO 16684-1 XMP Data model
https://www.iso.org/standard/75163.html
ISO 16684-2 XMP schemas using RelaxNG
ISO 16684-3 JSON-LD serialization of XMP
ISO 16684-4 Use of XMP for semantic units
RFC 7995 PDF Format for RFCs
Adobe
https://www.adobe.com/products/xmp.html