Perhaps the "D" stands for "Double", since two x16-sized PCI-E slots are included on board, supporting two PCI-E video cards. Don't be confused though; this is NOT an SLI board. For official SLI support, you'll need to look into the Lanparty UT NF4 SLI-DR. The SLI-DR is identical in every way, except that the chipset is the "SLI" version of the NF4, and therefore you get "official" SLI support.
I got it from here.
Thanks