I believe, though I can't find an unambiguous reference, that a tool or user is allowed to define and populate any XMP/Exif attribute it wants, within the techn

I believe, though I can't find an unambiguous reference, that a tool or user is allowed to define and populate any XMP/Exif attribute it wants, within the technical limits of attribute/value definition of the relevant metadata. Whether or not anything else can read and make sense of that metadata is a different problem.