domain vs subject area

domain and subject area both are nouns.

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

As nouns, subject area is a hyponym of domain; that is, subject area is a word with a more specific, narrower meaning than domain:

domain (noun)	subject area (noun)
the content of a particular field of knowledge	a branch of knowledge
people in general; especially a distinctive group of people with some shared interest
(mathematics) the set of values of the independent variable for which a function is defined
territory over which rule or control is exercised
a particular environment or walk of life