line of business vs product line

line of business and product line both are nouns.

Word	Noun	Adjective	Verb	Adverb
line of business	Yes	No	No	No
product line	Yes	No	No	No

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

line of business and product line: a particular kind of product or merchandise

Other synonyms of line of business include business line, line, line of merchandise, line of products.

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