There are a couple of versions of RDL. Americans tend to use that term to describe deadlifts that don't touch the ground. Europeans tend to mean a version of the deadlift where you pull from the ground with relatively straight legs.
Regular stiff-leg deads are usually done with legs as straight as possible, starting from the up position and not touching the ground between reps.
Ed Coan does stiff-legs pretty much the same way Europeans do RDL's, but he also starts with the bar a lot more forward and not touching the shins.