When blood pressure is measured, why must the arm cuff be held at the level of the heart?

Whenever a specific liquid fills in a container, and if the measurement of liquid pressure starts from the free liquid's surface and goes down into the container, then the value of pressure would be increasing.

If the measurement of pressure starts at a specific datum point and goes upward into the container, the pressure would decrease.

Generally, blood pressure measures the pressure of the blood coming out of the heart. Whenever the position of the cuff is below the heart level, then the blood pressure will not be the actual one; it shows the addition of the pressure from the pumping of the heart and the pressure due to the height of blood above the cuff. Due to this reading will be high. Likewise, if the cuff is positioned above heart level, the pressure will be too high because the measured pressure will be the sum of the pressure from the heart pumping and the pressure due to the blood's height above the cuff. Due to this, the reading will be too low.

So, the arm cuff must be held at the level of the heart to get the accurate reading.