That means that if ABS and PVC are connected together, some type of adapter or fitting needs to be used. That's it, that's all that's allowed. The image below shows one such adapter on the right side.
The Family Handyman on MSN

How to Glue and Join PVC Plastic Pipe

Gary Wentz, an editor for Family Handyman, will show you how to glue PVC pipes to get a fast, leak-free seal. Make an ...