dBmi Meaning in Testing
Without knowing what is the DUT and where the power level should be SdBmi, I guess:

You have a precision signal generator as your transmitter, it's connected to an antenna A  which should be at certain distance from your receiving antenna B. Antenna A has gain XdB. You direct A and B against each other, input to A power (S-X)dBm and check if the machine which is connected to B, works as wanted. I assume B is part of the device under test.