The diaphragm isolates the sensor from the process while transmitting the applied process pressure to the fill fluid, which in turn transmits the applied process pressure to the sensor.
The sensing/measurement range of any given commercial DP transmitter is usually stamped on the ID tag, but can be decoded from a part number, where one of the digits or characters in the part number represents the differential range.
So-called "smart" transmitters can be re-ranged over a turndown range using a communications protocol, like HART.