It is a standard way to measure it. So, you might just say that it measures "blood pressure on the upper arm" and that it is taken as a measure. Like temperatur

It is a standard way to measure it. So, you might just say that it measures "blood pressure on the upper arm" and that it is taken as a measure. Like temperature with a thermometer. Like, is the air pressure/temperature/contamination everywhere the same around my city? Well, obviously not, but we make do with what we have.