Volume units will not be sufficient because a cubic foot can have more or less steam depending on whether it is under more or less pressure. A certain flow of steam at 100 cubic feet per second can have dramatically different amounts of steam depending on the pressure and temperature. With mass flowrate units, the users can account for the amount of steam circulating in pounds rather than cubic feet of steam. This is desirable because the heating value of steam is related to the mass of the steam, not the volume alone. Standard units can be related to both volume and mass units.
Standard units are volume units at a certain pressure, temperature, and compressibility.
These units are used only with compressible gasses, such as natural gas. The base conditions ensure that every volume measured has the same amount of product in it. In other words, a cubic foot of natural gas at standard conditions will always have the same density. Because the density at standard conditions is constant, it can be multiplied into the standard flowrate to get mass flowrate. Make sure the standard units used are consistent throughout the calculations.