USV1, USV2 (Cut Valve)
WARNING: This page is about a different car, the 2009 Nissan Maxima. However, it is still accessible from the selected car via links, so may be relevant.
The cut valve shuts off the normal brake fluid path from the master cylinder, when VDC/TCS is activated.
HSV1, HSV2 (SUCTION VALVE)
The suction valve supplies the brake fluid from the master cylinder to the pump, when VDC/TCS is activated.