xpPPCollisions Object tab menu settings.
If enabled, particles will collide with other particles from the same emitter.
xpPPCollisions is disabled on the left and enabled on the right, where particle to particle collisions can be seen.
The bounce controls the speed with which the particle will rebound from the object.
A bounce of 100% means that the speed after hitting the object is the same as before hitting it.
These settings add friction between the particle and colliding surface.
Friction will cause rolling particles to come to a halt.
The higher the setting, the more quickly they stop.
You can add some random variation to the amount of friction with the Variation setting.