A certification check that each setting runs at a frequency the device was designed to produce. A reading that lands on the power-line frequency may come from the building's wiring rather than from a designed pulse. Gauss Labs checks every setting's measured frequency and flags any that sit close to the local mains frequency. See Power-Line Interference.
A building's wiring runs at 50 or 60 Hz. A setting that lands on one of those values may be picking up the power line rather than producing a designed pulse, so it is flagged.
