This is an optional Novarossi Complete Clutch Spring Kit, intended for use with the optional Novarossi Complete Clutch System (NVR71000). This kit includes four springs of each rate; 0.9, 1.0 and 1.1mm.
The stiffer the spring, the later the clutch will engage allowing the engine to spool up to a higher RPM range before the clutch comes on. But keep in mind, the later the clutch engages, the more the clutch will slip causing the clutch shoes to wear out more quickly.
Softer clutch springs allow the engine to engage earlier providing a smooth engagement which can be useful if the traction is very low.