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.