line of business vs field of operation

line of business and field of operation both are nouns.

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

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

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

Other synonyms of line of business include field.

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