What is the difference between Solid Core PVC pipe and PVC schedule 40 pipe?

http://www.charlottepipe.com/Documents/InTheNews/CPF_Value_Engineering_Brochure_R2.pdf

Cellular Core PVC Pipe:
Cellular or foam core PVC pipe conforming to ASTM F 891 is produced by
Extruding a layer of foamed PVC between two layers of solid PVC, thereby
Reducing the weight and cost of the product. Cellular core pipe is designed
For drainage (non-pressure) service only. The stiffness of cellular core pipe
In most diameters is lower than that of solid wall PVC pipe. Pipe stiffness
Is a key factor in determining the ability of pipe to resist external loads such
As earth or live loads in underground installations. For this reason many
Specifiers believe that solid wall PVC pipe is more robust and better suited
To the rigors of commercial construction.