“Restrictive Term, Element, Clause: A phrase or clause that limits the essential meaning of the sentence element it modifies or identifies. Professional athletes who perform exceptionally should earn stratospheric salaries. Since there are no commas before and after the italicized clause, the italicized clause is restrictive and suggests that only those athletes that perform exceptionally are entitled to such salaries. If commas were added before who and after exceptionally, the clause would be nonrestrictive and would suggest that all professional athletes should receive stratospheric salaries.”
Excerpted from: Strunk, William Jr., and E.B. White. The Elements of Style, Fourth Edition. New York: Longman, 2000.