Options Panel
To bring up the Options Panel, click the Options button in the Effect palette. Inside the Options panel, there is a single setting for the particle system.
Shadowlet Light Name
The Shadowlet Light Name field works with the Shadowlet group and After Effects' composition lights. When a comp light has the same name as the Shadowlet name, that light will be used to control the location of the Shadowlet's shadow.
Enter a name in the Option field, and use that same name for the light layer(s) in your Timeline. This will tell the light to recognize the Shadowlet.
By default, this Light Name is named 'Shadow' and that is an easy standard to use. But the Light Name can be changed to anything as long as the light's layer name matches. You can point to multiple lights with this method, naming them 'Shadow 1', 'Shadow 2' and so on. More info on the Shadowlet group page.
At left, Options uses the default 'Shadow' name. At right, it sees a light layer named 'Shadow'.