I don't see the phrase "MUST exist" anywhere in 3501? I see the word "exactly" in two sentences, neither of which appears to be the one you mean. Can you provid

I don't see the phrase "MUST exist" anywhere in 3501? I see the word "exactly" in two sentences, neither of which appears to be the one you mean. Can you provide a reference? I don't want to argue about the disadvantages of flags vs. mailboxes. It's a valid subject but I don't want a subject change.