army unit vs platoon

army unit and platoon both are nouns.

Word	Noun	Adjective	Verb	Adverb
army unit	Yes	No	No	No
platoon	Yes	No	No	No

As nouns, platoon is a hyponym of army unit; that is, platoon is a word with a more specific, narrower meaning than army unit:

army unit: a military unit that is part of an army
platoon: a military unit that is a subdivision of a company; usually has a headquarters and two or more squads; usually commanded by a lieutenant

army unit (noun)	platoon (noun)
a military unit that is part of an army	a military unit that is a subdivision of a company; usually has a headquarters and two or more squads; usually commanded by a lieutenant
	a team of policemen working under the military platoon system
	a group of persons who are engaged in a common activity