Veeco MS40 Operation And Maintenance Manual page 166

G1 is Gross Mode Gain.
G2 is Reverse mode gain.
G3 is Direct Flow gain.
G4 is Sniff gain.
G5 is Power Sniff gain.
G2 & G3 are set by computer.
G2 & G3 gains cannot be changed in Standard mode.
G1, G4 & G5 may be changed in Standard mode
during leak testing in the Gross mode or Sniff modes.
Can change any gain in Service mode.
1109-186-00 Rev. C f-iv