Supplemental Brake Assist
WARNING: This page does not describe the selected car, but rather 12 other vehicles, including the 2003 GMC Yukon XL, 2003 GMC Yukon, 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, and 2003 Chevrolet Tahoe. However, it is still accessible from the selected car via links, so may be relevant.
The Supplemental Brake Assist (SBA) unit creates vacuum for the brake booster in case of a vacuum loss or low vacuum to the brake booster.