Either your unit can be supplied in different ways. Or they sell different sensors with different power options.

In the case of 4-20mA - you send a voltage in the range 9-32 V to the unit.

The unit will then dynamically adjust its internal impedance to consume between 4 and 20mA.

You take a reading by measure how much the unit consumes.

This means that when the unit is bus-powered, the electronics in the device must consume less than 4mA, and the electronics will then burn an additional current of 0-16mA depending on measurement.

So you may supply 9V and must support up to 20mA.
Or you may supply 20V up to 20mA.
Or all the way to 32V up to 20mA.

Obviously, the amount of power needed from your USB connector will increase if you select a higher voltage.

But a higher voltage will allow a longer cable without problems with voltage loss in the cable. At 9V, you don't have any margin for voltage loss in the cable.