"IPTC:*" is the canonical OIIO naming scheme for all IPTC data that does not correspond to an existing global OIIO name (such as "Keywords").  There's an appendix in the OIIO documentation that gives all the supported IPTC fields (though I have not yet modified it to include the new fields I just added. I'll get back to that).

"Rating" is arbitrary metadata.  JPEG doesn't support arbitrary metadata, so it would get dropped on the floor.  But if you were writing to OpenEXR, it would output a "Rating" metadata.

"IPTC:Rating" is the IPTC Rating and corresponds to the "xmp:Rating" in the XMP representation of IPTC data. There is no IIM tag for it, as far as I know.

"IPTC:Urgency" is also supported and is quite separate, it corresponds to the "photoshop:Urgency" field in XMP representation and tag 10 in the IIM representation.