RFD vs postal service

RFD and postal service both are nouns.

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

As nouns, postal service is a hypernym of RFD; that is, postal service is a word with a broader meaning than RFD:

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

Other hypernyms of RFD include mail, mail service, post.

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