A/T: System Description

WARNING: This page is about a different car, the 2011 Nissan 370Z. However, it is still accessible from the selected car via links, so may be relevant.

When selector lever is P or N, power is supplied to starter relay and starter control relay by TCM. And BCM and IPDM E/R (CPU) detect selector lever P/N condition by the inputted signal.
When starter operating condition is satisfied, IPDM E/R turns starter control relay ON by starter control relay control signal.
When engine cranking condition is satisfied, BCM turns starter relay ON by starter relay control signal.
Then battery power is supplied to starter motor ("S" terminal) through starter control relay and starter relay. And IPDM E/R (CPU) detect starter relay condition by the inputted signal.