Because The superlift lowers your torsion bar while hte RCD converts to coils and installs radius arms (which kind of mimic the position of torsion bars but not as bad..
So, additionally from there, some people add coilovers to the superlift. TECHNICALLY you can remove the torsion bars completely and you don't actually need radius arms therefore the Superlift would create an RCD with no radius arms.
To sum it up:
RCD: Radius arms and coilovers
Superlift: tbars(suck) and shocks
modified superlift: just coilovers.