SLDL: Starts from floor or below (ie standing on mats, box, etc)
RDL: Starts from the hang.
I always thought of the RDL as a movment specifically designed to strengthen the second pull in an o-lift, so range of motion would stop somewhere around the knee or just below. Alot of the times I will start from the hang...BUT...I am standing on mats and take the bar down to the top of my feet....so that particular lift has traits of both the RDL and an SLDL.
For overall strengthening of the posterior chain or to strengthen the deadlift, I like an SLDL pulled from the floor. If I start from the hang it is generally because I was being a lazy ass and a loaded bar was already sitting in a rack somewhere in the gym and I didn't feel like setting a new bar up on the floor.