compartment vs PO Box

compartment and PO Box both are nouns.

Word	Noun	Adjective	Verb	Adverb
compartment	Yes	No	No	No
PO Box	Yes	No	No	No

As nouns, PO Box is a hyponym of compartment; that is, PO Box is a word with a more specific, narrower meaning than compartment:

compartment: a space into which an area is subdivided
PO Box: a numbered compartment in a post office where mail is put to be called for

Other hyponyms of compartment include cell, cubbyhole, pigeonhole, call box, letter box, POB, Post-Office box.

compartment (noun)	PO Box (noun)
a partitioned section, chamber, or separate room within a larger enclosed area	a numbered compartment in a post office where mail is put to be called for
a space into which an area is subdivided