(per Doug W5WC)
The 0dB and -20dB sets the attenuator on the receiver. The -10db, -20db, and -30db controls the attenuator on the HPF section.
The -40db and -50db is a combination of the attenuator on the receiver and the one on the HPF board. Note the small letters used for the 'db' in the second -20db setting.
All of the receivers based on the OpenHPSDR design have a 20dB ADC driver (preamp) that is hard wired to be ON full time.
The attenuator 'ATT' and the step attenuator 'S-ATT' controls set the 31dB step attenuator on the Hermes, Angelia, and Orion boards. The Alex filter set and the ANAN 100W PA Boards have a 10dB and a 20dB fixed attenuator that are also controlled by the 'ATT' and 'S-ATT' controls.