Thats actually some typo, but the ST-RR is based from the inferno buggy, just made up with parts to make it a real truggy. It uses some of the same parts as a 777, but has the mechanicals parts for a truggy, thats all.
As for new cars, the ST-RR is the newest model of the two. Of course the ST-R was the older model, but can be changed over to a ST-RR also. For top RC cars to maintain there racing competitiveness, they will have to update or change models on a regular basis.