code/game/objects/effects/particle_holder.dm

/obj/effect/abstract/particle_holder	objects can only have one particle on them at a time, so we use these abstract effects to hold and display the effects. You know, so multiple particle effects can exist at once. also because some objects do not display particles due to how their visuals are built
/obj/effect/abstract/particle_holder/cached	A subtype of particle holder that reuses the same particles to reduce client lag when rendering certain atoms, usually found in large quantities and close together. Since it reuses the same instances, modifying an instance of particles will affect all atoms that show it, therefore procs like set_particle_position() shouldn't be used here.