In this case this unit only support 19 V DC external and internal, you can see more detailed information in the technical product specifications.
See page 43
I don't mean to be contrary, but my DC3217IYE runs just fine off of 12v. I have most NUC models and other than the DN2820 they all power up with either the 12v power adapter or the 19v power adapter. The DN2820 was designed specifically for 12v so 19v would cook it, but the other models run fine on 12v.
My .02 cents.
Having said that, I assume running it off of 12v voids the warranty if there are any issue with the board.
Thanks jasonNUC for the information you kindly shared with us.
The voltage we recommend is what is showing in the technical specifications but if you are able to use 12V your free to do it, probably nothing will go wrong but the unit has been tested and validated to use 19V.