While I don't have a current, controlled copy of this standard, I believe it has traditionally been for rigid (i.e. mostly unplasticized) pvc pipe. I also believe rigid pvc has at least traditionally been the most common of the pvc compounds used for sewer service in the USA, and gravity pipes have sometimes been compounded with even higher levels of "fillers" (such as calcium carbonate/talc) than pressure pipes, that may increase at least the short-term stiffness and lower at least the manufacturing cost of the pipe. I like you will await more learned response.