fatigue duty vs duty assignment

fatigue duty and duty assignment both are nouns.

Word	Noun	Adjective	Verb	Adverb
fatigue duty	Yes	No	No	No
duty assignment	Yes	No	No	No

As nouns, duty assignment is a hypernym of fatigue duty; that is, duty assignment is a word with a broader meaning than fatigue duty:

fatigue duty: labor of a nonmilitary kind done by soldiers (cleaning or digging or draining or so on)
duty assignment: a duty that you are assigned to perform (especially in the armed forces)

Other hypernyms of fatigue duty include assignment.

fatigue duty (noun)	duty assignment (noun)
labor of a nonmilitary kind done by soldiers (cleaning or digging or draining or so on)	a duty that you are assigned to perform (especially in the armed forces)