Based on the typical terminology used in the modding community, "solid feature" is likely referring to a specific coding syntax or a modding concept: Solid Parts (versus Flexible parts).
Installing SLRR mods isn't as easy as Steam Workshop, but follow these steps: Street Legal Racing Redline V2.3.1 Mods
If you want a "plug and play" experience, look away. If you enjoy spending three hours reading a forum thread to understand why your V8 swap is causing the game to crash to desktop upon loading the freeway map, you will find a home here. Based on the typical terminology used in the