Transparency depth
This option in some of the Huygens Volume Visualization tools (those that use Iso Surfaces) controls how different surfaces are seen through the others. The effects are mainly visible when you have some objects intersecting with other. With the Simple depth you can only see through one surface, the surface closest to the viewer. Quite often this is sufficient.
With the Normal depth, one can see through up to two surfaces. Thus, one object B inside the surface A will appear be more visible than a third object C behind A because B is only screened by the piece of surface A closer to the viewer's eye, while the object C is screened by two pieces of surface A.
The 'Deep' depth option will consider many more screening levels, making the final rendering computationally more complex.
When the transparency depth of the first Graphics Pipe is set to zero, the transparencies of the other pipes are not taken into account anymore. This is because the data in those pipes are inside or behind the primary one.