There are two topics running in this thread: (1) how to set up an instrument per the manufacturer's instructions, and (2) how direct process measurements are used to compute a derived measurement.
In the case of #1, regarding the Endress & Hauser Prowirl 200 vortex flowmeter, one can purchase application options such that the instrument will compute derived measurements from multiple inputs. For the derived measurement to be accurate (within capability of the instruments and estimation technique), the instrument must be set up to correctly convert those external inputs to values used by the estimation technique. This is where it is essential for the external scaling and units to be agreed upon by the producer (pressure transducer) and consumer (enhanced flow meter).
The second topic seems related to computing derived measurements -- in this case mass flow. Since the Prowirl 200 is a vortex flowmeter, its base measurement is volumetric (actual) flow. Using external inputs such as temperature and pressure, it will compute estimates for values such as standard flow, mass flow, and energy flow. These estimates are based on industry standards, using equations requiring the external inputs. The equations will use measurements in engineering units based on the standard, and may require an internal conversion (e.g., psig to psia) to match the standard. So the manufacturer's choice of external input units is somewhat arbitrary as long as the values are internally converted consistent with the expression of the standard.
In the case of mass flow of steam, the Prowirl 200 specifies that it uses the IAPWS-IF97/ASME standard. Further investigation down this path will show that fluid density is the critical requirement, and depends on temperature and pressure (if superheated).