Damage Reduction — DR/evil vs DR/unholy [closed]

we’ve got a party with a Celestial Mystic (Book of Exalted Deeds), and I was reading about his level 9 ability, which grants him DR 10/Unholy. I got confused because it’ll usually be DR/good or DR/evil.

Would DR/unholy be different from DR/evil, as in it would only be breached by unholy weapons, and not attacks from evil outsiders? Or is this a typo of some sort and it should be DR 10/evil?