XII vs boxcars

XII and boxcars both are nouns.

Word	Noun	Adjective	Verb	Adverb
XII	Yes	No	No	No
boxcars	Yes	No	No	No

As nouns, boxcars is a hyponym of XII; that is, boxcars is a word with a more specific, narrower meaning than XII:

XII: the cardinal number that is the sum of eleven and one
boxcars: (usually plural) an expression used when two dice are thrown and both come up showing six spots

XII (noun)	boxcars (noun)
the cardinal number that is the sum of eleven and one	(usually plural) an expression used when two dice are thrown and both come up showing six spots