Screenshot attached about the display and a short explantion is here:
ID2: id number (2) of the test
DR2: data rate (2) of the last message
P14: power of the last transmission, 14dBm, this is the maximum you can use in Europe
Up Margin 0: this value describe device->gateway direction transmission
DWN -121 db -12.2: these values describe gateway->device direction transmission
-121 is the RSSI value, what it means is the receive level on device side. minimum value is somewhere around -135 dBm
-12.2 is the SNR value, and what it means is the ratio between the signal and the noise. minimum value is around -20 dB
You can change DR (data rate) and P (power) by hitting SW1
i recommend you to do testing when P is 14 and DR is 0 or 5. You will cover most cases with these settings. Survey was successful when you see some values on the screen. If you want to know how good the coverage is then you look for RSSI (which is the first value after DWN), bigger is better here.
this is valid for Survey single mode.
Survey sweep mode will do 28 tests and change automatically DR and P values.
NOTE: we are using radio here and 100% success is never guaranteed even when the gateway is literally next to you, this is why i also recommend you to do multiple measurements from one spot to check when there is connectivity or not.