Circuit/System Description
WARNING: This page is about a different car, the 2020 Cadillac XT5. However, it is still accessible from the selected car via links, so may be relevant.
The solenoid valve directs pressurized fluid to exit the accumulator to apply the clutches for a 1st gear start, during an Auto Start event.
For an overview of the component/system, refer to Park - Engine Running (Hydraulic Circuits) , and Park - Engine Running (Mechanical Powerflow)