This filter works only when "Use Attrib0" is NOT selected in Classification option.
(If so, all particle information is poured to graphic board, without checking each particle information).
With this filter, all particles are divied to the boxes defined by boxnum parameter, and marked according to the box id.
Then, these box is tested whether within the sight or not, and particles marked with ids of boxes outside the sight is skipped in rendering process.
Boxnum parameter is resolution of the boxes for x,y,z direction. Too much boxes makes overhead for cheking the boxes within or without the sight too large.
From experience, somewhere between 8 to 12 is good.
The definition of boundary is commonly used in Data Loader Dialog and Copybox Filter.
For this filter to work properly, the boudary box must contains all particles, like following image.
This filter works especially well if you are making fisheye images with camera within the simulation data (looking outside from inside).
In this case, left half of particles are out of sight when rendering right part, and vice versa.