Good question, I also find the range of suppliers offered by Rockauto bewildering.
So it'd be useful if someone had a guide/rule of thumb.
I've never ordering anything from them yet either, so would be good to get to grips with it before I place an order.
Update: just found this on their website; http://www.rockauto.com/help/?page=2#WHATDIFF, so they group products into Economy, Premium, etc. Looking at each parts Info screen I can see warranty levels, so that's an additional help. Seems like I just need to get familiar with their website...