frontier vs subject area

frontier and subject area both are nouns.

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

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

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

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

frontier (noun)	subject area (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