Here’s the rule of thumb: use that if the clause it introduces contains information that is essential to the meaning of the sentence and which if the clause contains extra or descriptive information. Grammarians term these essential clauses restrictive and the inessential ones nonrestrictive; a nonrestrictive clause is set off by commas. Beware — the line between essential and inessential information can be a very fine one. Test by trying the sentence for sense without the clause in question, but recognize that sometimes it’s simply a judgment call.