Hi Vincent,
Max input directly controls the sensitivity of the receiver. For the best sensitivity max input should be set to ~5 dB above the maximum possible input level. The receiver can clip signals at or above the max input level. The software will warn you with a signal is clipped.
Ref offset is a convenience control which adjusts the sensitivity of the receiver to account for external attenuation/gain. IE, if you attenuate the input signal by 10dB (pad), setting the ref offset to +10dB will account for this and adjust the receiver sensitivity accordingly.
Let me know if you have additional questions.
Regards,
Andrew