What is zero elevation in a transmitter

Explain the zero elevation in a transmitter

Elevation and suppression comes for the level transmitter in the tank where installation of the transmitter depends on the access and maintenance purposes

So in closed tank Transmitter installs below high tapping point then compensation of high and low port is called zero elevation to get the real level of the tank.

And in suppression compensation only at high port done because its open tank low port open to vent