EyeonTips:Manual/Fusion 6/pCustom

From VFXPedia

There are more particle tips in the Tips & Techniques section.

To better visualize what the particle ID does, create a pEmitter with the emission rate set to 1 per frame. In a pCustom tool enter id/50 as red expression and if(id=4,0.3,0.1) as size expression. (source: this pigsfly post by Gringo)

In case you are looking for the docs for pCustomForce, hopefully this tip will show up in your search. The controls for pCustomForce are very similar to those of pCustom.