Okay, I'll try to explain this as clearly as possible.
Recap; we have three discs : Las Vegas (shot by fan), The Gift (official NIN.com footage) and Bonus.
The Gift, being one hundred percent HD sourced, will be available as a Blu-Ray.
Las Vegas, being shot by fans, includes a lot of SD (which stands for Standard Definition) footage, actually more than half, which makes it pointless to author a Blu-Ray: upconverting to HD only makes SD footage look grainier, more pixelated, uglier. We do have the possibility to do an HD version of Las Vegas but it's not currently in the plans.
Bonus disc has also a lot of SD content, hence no Blu Ray in the plans for now.
DL and SL means
Dual Layer (DVD R DL) and
Single Layer (DVD R).