Restricitive Clauses limit the possible meaning of a preceding subject. [uses that, no comma]
Nonrestrictive clauses tell you something about a preceding subject, but they do not limit, or restrict, the meaning of that subject. [uses which, with comma]
- Correct Restrictive Use: The store honored the complaints that were less than 60 days old. --> the store honored only those complaints less than 60 days old, but not those over 60 days old.
- Correct Nonrestrictive Use: The store honored the complaints, which were less than 60 days old. --> the store honored all the complaints, all of which were less than 60 days old.