high relief vs relievo

high relief and relievo both are nouns.

Word	Noun	Adjective	Verb	Adverb
high relief	Yes	No	No	No
relievo	Yes	No	No	No

As nouns, relievo is a hypernym of high relief; that is, relievo is a word with a broader meaning than high relief:

high relief: a sculptural relief in which forms extend out from the background to at least half their depth
relievo: sculpture consisting of shapes carved on a surface so as to stand out from the surrounding background

Other hypernyms of high relief include embossment, relief, rilievo, sculptural relief.

high relief (noun)	relievo (noun)
a sculptural relief in which forms extend out from the background to at least half their depth	sculpture consisting of shapes carved on a surface so as to stand out from the surrounding background