D&D3.5e — Damage Reduction — DR/Unholy vs DR/Evil

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. Would this be different from DR/evil, as in it would only be breached by unholy weapons, and not attacks from evil outsiders?

I’m asking this because I haven’t seen any monsters or anything that have things like DR/Unholy, Holy, Axiomatic, etc… it’s usually just Good/Evil/Lawful/Chaotic. Has anyone seen any material throughout the books like this? My intuition tells me it’s a typo and should be DR 10/evil, but the errata doesn’t say anything about this.