subaltern vs junior-grade

subaltern is a noun but junior-grade is not a noun.

Word	Noun	Adjective	Verb	Adverb
subaltern	Yes	Yes	No	No
junior-grade	No	Yes	No	No

As adjectives, subaltern and junior-grade are synonyms defined as:

Other synonyms of subaltern include lower-ranking, lowly, petty, secondary.

subaltern (noun)	junior-grade (noun)
a British commissioned army officer below the rank of captain

subaltern (adjective)	junior-grade (adjective)
inferior in rank or status	inferior in rank or status