Main difference is how deep you go. Stiff Leg Deadlift you allow your back to round a bit at the bottom, might even stand on a platform to really emphasize this. Romanian Deadlift is more of a hamstring exercise where you're emphasis is keeping your back straight and forcing your butt backwards.
Ideally SLDL is completely straight though not locked out - never lock out that would just cause injury. RDL the knee bends to allow your butt to go backwards.
Image from: Romanian Deadlift vs. Stiff Legged Deadlift
Another subtle distinction that often occurs is that SLDL the bar is generally a bit more forward where RDL's it will be right against your legs, sometimes even use Dumbbells by the side of your legs. Can see that too in the above depiction where the bar is above her toes on the SLDL but closer to her ankle in the RDL.
Personally, I never ever do Stiff Legged Deadlifts. Romanians however are my go-to hamstring and glute exercise.