Post-Office box vs compartment

Post-Office box and compartment both are nouns.

Word	Noun	Adjective	Verb	Adverb
Post-Office box	Yes	No	No	No
compartment	Yes	No	No	No

As nouns, compartment is a hypernym of Post-Office box; that is, compartment is a word with a broader meaning than Post-Office box:

Post-Office box: a numbered compartment in a post office where mail is put to be called for
compartment: a space into which an area is subdivided

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