Rule 3. Which introduces a nonessential clause (also known as a nonrestrictive or nondefining clause), which adds supplementary information.
Example: The product claiming "all natural ingredients," which appeared in the Sunday newspaper, is on sale.

Rule 4. If that has already appeared in a sentence, writers sometimes use which to introduce the next clause, whether it is essential or nonessential. This is done to avoid awkward formations.
Example: That which doesn't kill you makes you stronger.