You can find out all about Light Filters in the Renderman documentation.
Adding Light Filter
To add a light filter, in your OBJ pane, select the light filter in the TAB menu.
Parent the light filter OBJ to the light OBJ if you want the filter to inherit the transformation from the light.
We can translate the light filter and scale it. It is best to translate the light filter in Z away from the light to avoid seeing the edge of the light filter.
Set Light Filter
Drag your light filter VOP to the Light Filter parameter of your light.
Using Multiple Light Filters
A light can have more than one light filter.
For multiple light filters, in your RIS Network, create a PxrCombinerLightFilter VOP.
In the combiner, hit the + icon to dynamically create a light filter. Use the x icon to remove it. This combiner groups all the light filters under specific combining operators. That is, all the light filters under "Mult" will be multiplying by each others while "Max" will take the maximum of the light filters in the max group.
You cannot wire a Pxr light filter combiner into another Pxr light filter combiner.