"That" vs. "Which"
Many grammar books advise readers to observe distinctions between that and which. This book will do the same, but with a caveat: regardless of what is written here, you could close this guide and easily find any number of excellent books written by excellent novelists and nonfiction writers who do not consistently observe the distinction outlined below. Especially in business, though, the difference is worth observing, for it helps ensure clarity.
Now, imagine you have a business partner with whom you are having a little disagreement over money. Which of the following statements would you prefer to hear from your partner?
I will give you all the money, which I owe you.
I will give you all the money that I owe you.
Should you ever confront such a situation, you might want to opt for a comma and a which. Which introduces nonessential information. That doesn’t mean the information that follows is irrelevant; it simply means that you could remove the clause which I owe you without destroying the integrity of the sentence. The first sentence says essentially, “I will give you all the money. And, oh, by the way, I happen to believe I owe it to you.”
The second sentence, on the other hand, means that the speaker will hand over only the money that he feels he owes you. Out of, say, $ı million, that might be $2, or it might be $990,000. The word that introduces essential information that cannot be removed without destroying the integrity of the sentence. In this case, it also leaves you financially vulnerable! For additional discussion of the that–which distinction, including punctuation guidance, see Section 3.ı.6.