communications vs subject field

communications and subject field both are nouns.

Word	Noun	Adjective	Verb	Adverb
communications	Yes	No	No	No
subject field	Yes	No	No	No

As nouns, subject field is a hypernym of communications; that is, subject field is a word with a broader meaning than communications:

communications: the discipline that studies the principles of transmitting information and the methods by which it is delivered (as print or radio or television etc.)
subject field: a branch of knowledge

Other hypernyms of communications include bailiwick, discipline, field, field of study, study, subject, subject area.

communications (noun)	subject field (noun)
the discipline that studies the principles of transmitting information and the methods by which it is delivered (as print or radio or television etc.)	a branch of knowledge