This read-write integer, valued 1 to 3, configures the allowed frequency deviation tolerance the ATS accepts from a power source before considering it out of range for transfer purposes. Admins tune this alongside the voltage-range setting to fully define what counts as an acceptable versus unacceptable power source, since a source can be frequency-unstable even while voltage stays fine. Setting this tolerance tighter, a lower value, on an ATS backing frequency-sensitive lab equipment ensures even minor frequency drift on a generator source triggers a transfer to the more stable feed.