DIPS (Ductile Iron Pipe Size) was originally used to specify the size of ductile iron pipe and is still used to designate that pipe today. Other types of pipe, primarily PVC and PE pipe use the same DIPS sizing designation to classify them for use with compatible fittings, valves, and appurtenances made to the same sizing standard.
DIPS is a pipe outside diameter control. It is important to note that DIPS sized pipe is only compatible with DIPS sized valves, fittings and appurtenances.
IPS controlled pipe, on the other hand, can be used DIPS valves, fittings, and appurtenances (with special gaskets and backing rings). This is due the smaller OD sizing of IPS pipe