The details relating to the nominal size of devices used in fluid technology are key variables which do not refer to units or dimensions and do not have to be associated with any numerical variable for these devices.
Specifically, the nominal size for pumps and motors as per ISO 3662 is equivalent to their geometric displacement (e.g. nominal size 88 = 88 cm3).
There are no standard specifications for valves. It has become normal practice to link the nominal size with the size of the connecting bore. However, it tells us nothing, for example, about the maximum or permissible flow. It is in this context that the nominal size is used in standards such as DIN 24 340 too.