Complex sentence: This time, the series was more restricted to current models that is being available in dealerships throughout the US, other than that, the series had rules and race format that were similar to the RS series, being an endurance series.
Split into simpler sentences: This time, the series was more restricted to current models, that is to say models then currently available for retail sale in dealerships throughout the US. Other than that, the series had rules and race formats that were similar to the RS series, being an endurance series.