See Preparing for Installation and Power On.
In Standby power mode, the host is powered off but the SP is powered on. The power supplies are connected to a power source, the AC OK power supply indicators illuminate, the SP OK indicator is steady on, and the System OK status indicator flashes slowly.
The front panel includes buttons for System A and System B. System B is only present on dual 4-socket systems.
The host boots and the server enters Main power mode. The System OK indicator goes steady on when the host is fully booted.