XMP property `crs:CropUnits` is declared as `Integer` but it could be better `Closed choice of Integer`

[ceztko]

Present since XMP Specification 2005, and still in the 2022 revision, crs:CropUnits is declared as an Integer but in the ends the specification enumerates all the possible values, so it should be better Closed choice of Integer as done for a many other properties, most of them in exif or tiff namespace, eg. tiff:ResolutionUnit. The issue is present both in the current specification pdf document or the Markdown rendition. Excerpt below:

This issue has been first created in the pdf-issues repository but moved to this one after the recommendation in the PDF TWG discussion.