xpScale Object tab menu.
Checking this box enables the xpScale modifier.
Animation to demonstrate particles being scaled up by the xpScale modifier.
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.
Set at Particle Radius, by default.