See the posts where that was discussed/clarified. I did initially think he was talking about AC for coil and contacts. There are issues on the coil side, a DC relay isn't designed to work with AC. And there are issues on the contact side too. The contacts can typically handle AC or DC, but are derated for less voltage and/or less current with DC. I provided a datasheet that shows a typical 10A rated relay contacts can be used with 250V AC, or 30V DC.