frontier vs subject field

frontier and subject field both are nouns.

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

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

frontier: an undeveloped field of study; a topic inviting research and development
subject field: a branch of knowledge

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

frontier (noun)	subject field (noun)
an undeveloped field of study; a topic inviting research and development	a branch of knowledge
a wilderness at the edge of a settled area of a country
an international boundary or the area (often fortified) immediately inside the boundary