You can use the Particle Collision Event Editor to create and edit events that occur when particles collide with other geometry. A collision event involves an original particle system, which is the object that executes an event when it collides with collision geometry, and a target particle system, which is the particle system that emits particles when the event is executed. You can specify an existing particle system as a target for the collision event. See Create particle collisions.
Using collision events in your nParticle effects lets you can combine particle systems, emit new particles into particle systems in the advanced frames of a simulation, and accurately kill particle systems when they collide.
To open the Particle Collision Event Editor, select Particles > Particle Collision Events Editor.