systems theory vs subject area

systems theory and subject area both are nouns.

Word	Noun	Adjective	Verb	Adverb
systems theory	Yes	No	No	No
subject area	Yes	No	No	No

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

systems theory: the interdisciplinary study of systems in general, with the goal of elucidating principles that can be applied to all types of systems at all nesting levels in all fields of research.
subject area: a branch of knowledge

Other hypernyms of systems theory include bailiwick, discipline, field, field of study, study, subject, subject field.

systems theory (noun)	subject area (noun)
the interdisciplinary study of systems in general, with the goal of elucidating principles that can be applied to all types of systems at all nesting levels in all fields of research.	a branch of knowledge