A blank destination is the document's own profile, so in an RGB document Press Quality (Preserve
Numbers, blank) converted CMYK colours and images to RGB. Output › Destination can now be
DOCUMENT_CMYK, the CMYK profile of the colour settings in effect whatever the document's colour
mode (Illustrator's "Document CMYK"); Press Quality uses it, and the Save PDF dialog lists it after
Document profile. Test: in an RGB document a CMYK image keeps its inks and an RGB image comes out
CMYK.