More than any other question we get from admin readers, we see this one: “How do I know when to use which and when to use that?"
Here’s the answer: To determine which to use, you’ll need to figure out whether the phase following “which” or “that” adds crucial information or simply additional information. “That” is correct in restrictive clauses; “which” in nonrestrictive clauses. When a comma can be inserted, use "which."
For example, consider these two sentences, which are subtly different:
The invoices, which are on blue paper, have been revised to reflect price changes.
The invoices that are on blue paper have been revised to reflect price changes.
We can tell that in the first case, the fact that the invoices are on blue paper is additional information. Whoever printed the invoices happened to print them on blue paper. The fact that they’re on blue paper isn’t crucial information from the writer’s perspective. If it weren’t included in the sentence, we’d still fully understand what the writer is telling us.
In the second sentence, the fact that the invoices are on blue paper is crucial information. The writer is telling us that the invoices on white or yellow paper do not reflect the revised pricing, and if we’re looking for those revisions, we’ll need to see the invoices on blue paper.
Which = additional information. That = crucial information.