I know I'm not the most experienced lifter here but I just started doing SLDL recently, and before doing so I tried to research as much as I can on it and on the differences between the RDL. This is the conclusion I came to from the informations I gathered:
The Romanian Deadlift doesn't start from the floor. You unrack the bar and slowly move the bar along the thigh while driving the hips back and making sure the lower AND upper back is kept locked into it's initial arch. With most people flexibility, this will usually bring the bar not much past the knees.
The SLDL starts from the floor, the bar being a couple inches away from your shins. On the descent, the SLDL allows for upper back rounding near the end of the movement (instead of keeping it locked in the initial position for RDL) which allows for a greater range a motion and to go all the way down to the ground.