The number of simulation sub-steps that occurred during the last simulation step.
When simulation sub-stepping occurs, the number of sub-steps that run are monitored with the actual count available for analysis here. The cost of sub-stepping should be monitored with the Unity Profiler using the Physics 2D Module.
See Also: Physics2D.useSubStepping, Physics2D.maxSubStepCount, Physics2D.minSubStepFPS and PhysicsScene2D.subStepLostTime.