General tab menu.
Checking this box activates xpSpawn.
Set at Independent, by default.
You can change this to Action-Controlled.
In this mode, particles will be affected if they come into the field of effect of the modifier.
When in the Action-Controlled Mode setting, the modifier will only act on a particle when told to do so by an action.
This is the emitter which will generate the spawned particles.
Without an emitter in this field you cannot access any of the other parameters and the modifier will have no effect.
In general, it is recommended that you use a second emitter for this purpose; it has the advantage that you can set the parameters of the spawned particles to be quite different from the source particles.