earnings vs combat pay

earnings and combat pay both are nouns.

Word	Noun	Adjective	Verb	Adverb
earnings	Yes	No	No	No
combat pay	Yes	No	No	No

As nouns, combat pay is a hyponym of earnings; that is, combat pay is a word with a more specific, narrower meaning than earnings:

earnings (noun)	combat pay (noun)
something that remunerates	extra pay for soldiers engaged in active combat
the excess of revenues over outlays in a given period of time (including depreciation and other non-cash expenses)