systems theory vs subject field

systems theory and subject field both are nouns.

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

As nouns, subject field is a hypernym of systems theory; that is, subject field 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 field: a branch of knowledge

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

systems theory (noun)	subject field (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