mail service vs RFD

mail service and RFD both are nouns.

Word	Noun	Adjective	Verb	Adverb
mail service	Yes	No	No	No
RFD	Yes	No	No	No

As nouns, RFD is a hyponym of mail service; that is, RFD is a word with a more specific, narrower meaning than mail service:

mail service: the system whereby messages are transmitted via the post office
RFD: free government delivery of mail in outlying country areas

Other hyponyms of mail service include airmail, airpost, snail mail, rural free delivery.

mail service (noun)	RFD (noun)
the system whereby messages are transmitted via the post office	free government delivery of mail in outlying country areas