guard duty vs duty assignment

guard duty and duty assignment both are nouns.

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

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

guard duty: the duty of serving as a sentry
duty assignment: a duty that you are assigned to perform (especially in the armed forces)

Other hypernyms of guard duty include assignment.

guard duty (noun)	duty assignment (noun)
the duty of serving as a sentry	a duty that you are assigned to perform (especially in the armed forces)