This reports the health of the AC power feeding into this module, distinguishing a fully healthy input, ok, a within-tolerance-but-degraded input, safe, an input present but not phase-synchronized to what the system expects, notSync, and no input at all, off. A module reporting notSync(2) specifically points to an out-of-phase or frequency-mismatched AC source feeding that module, which is a different problem, and a different fix, than a module reporting off(3) because its AC feed was simply cut, for example a tripped breaker. A technician troubleshooting one module out of several in the same phase reporting notSync(2) while its neighbors report ok(0) would suspect a wiring fault specific to that module's feed rather than a site-wide power problem.