Ignition Switch System: Acc
WARNING: This page is about a different car, the 2020 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.
When the ignition switch is in the ACC position, the switch routes voltage through the ACC-designated circuit to the BCM. The BCM interprets this as the ignition accessory mode.
The BCM communicates the ignition mode to the other modules by sending an ignition status message over the CAN and energizes the accessory designated relays and provides power to some electrical system.