I know that the course materials say that joining ABS and PVC is not allowed, but that is only a partial answer. I can’t speak for everywhere and every jurisdiction, but in everyone that I have worked in, it is allowed if you use a special transition glue - which is typically white.
It’s still not good practice, but it is allowed in Canada.
I am just asking because I would like to know if there anywhere it actually isn’t. Does anybody know?