field of operation vs line of business

field of operation and line of business both are nouns.

Word	Noun	Adjective	Verb	Adverb
field of operation	Yes	No	No	No
line of business	Yes	No	No	No

As nouns, field of operation and line of business are synonyms defined as:

field of operation and line of business: a particular kind of commercial enterprise

Other synonyms of field of operation include field.

field of operation (noun)	line of business (noun)
a particular kind of commercial enterprise	a particular kind of commercial enterprise
	a particular kind of product or merchandise