the black stuff is used for drains since it is less pressure... usually just gravity. the white stuff is used for distribution of water under pressure. sch 40 vs 80 has to do with the pressure being applied. 80 being thicker for more pressure. Both schedule 40 and 80 PVC are used widely around the world. Each one has its benefits in different applications. Schedule 40 pipe has thinner walls, so it is best for applications involving relatively low water pressure. Schedule 80 pipe has thicker walls and is able to withstand higher PSI (pounds per square inch). This makes it ideal for industrial and chemical applications. To give you an idea of the size difference, 1” schedule 40 PVC pipe has a .133” minimum wall and 450 PSI, while schedule 80 has a .179” minimum wall and 630 PSI.