Relay Controlled Power Mode
WARNING: This page is about a different car, the 2006 Chevrolet Monte Carlo and 2006 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
The body control module (BCM) uses the discrete ignition switch inputs Off/Run/Crank, Accessory, and Ignition 1, to distinguish the correct power mode. The BCM, after determining the desired power mode, will activate the appropriate relays for that power mode.