Use that to introduce a restrictive clause, which is a clause that is essential to the meaning of the sentence. Use which to introduce a nonrestrictive clause, which is a clause that you can omit without affecting the meaning of the sentence. If you can insert a comma before the clause, it is nonrestrictive.
Examples
The red dot indicates that you have new voicemail messages.
The available permissions are based on the company policy, which your company administrator sets.
