shoulder mark vs epaulet

shoulder mark and epaulet both are nouns.

Word	Noun	Adjective	Verb	Adverb
shoulder mark	Yes	No	No	No
epaulet	Yes	No	No	No

As nouns, epaulet is a hypernym of shoulder mark; that is, epaulet is a word with a broader meaning than shoulder mark:

shoulder mark: epaulets that indicate rank
epaulet: adornment consisting of an ornamental cloth pad worn on the shoulder

Other hypernyms of shoulder mark include epaulette.

shoulder mark (noun)	epaulet (noun)
epaulets that indicate rank	adornment consisting of an ornamental cloth pad worn on the shoulder