From LightWiki

Prior to 9.3, LightWave was naive in how it calculated refractions when lightrays exited a material. You had to use 'air polygons' or Switch nodes to handle exiting refraction correctly. LightWave 9.3 introduces a system called the volume stack, which basically handles exiting refractions automatically for you. There is a small price to pay, your models must be built correctly for the volume stack to work correctly.

There are two basic rules to follow.

1. All transparent surfaces that are to be processed by the volume stack should be single sided.
2. Every material to be handled by the volume stack should have it's outer surface modeled to completely enclose the volume. So if you were to model a glass and water inside, you would model the empty glass, and then model the outer surface of the liquid, including where it touches the sides of the glass, to completely enclose the liquid volume. <image here>