From Valve Developer Community
Allows for basic movement of the particle through space.
- Gravity to apply to the particle.
- Drag to apply to the velocity of the particle. Numbers should be under 1.0, with common ranges from 0.05 - 0.25.
- max constraint passes
- The maximum number of passes that will be attempted to resolve conflicts between multiple constraints interacting on a particle. Rarely will need to be altered.