The gold standard for BP measurement is direct measurement via an arterial catheter - an invasive and relatively risky procedure.

For a device to be approved as a Class 1 medical device for blood pressure measurement, the FDA wants +/- 3mmHg to an established BP measuring method.

Wow, I didn’t know that.

I still think though, that in a common setup the +-3mm is impossible to reach. People don’t sit perfectly still, may flex their muscles during the measurement, etc