Nylon Interface Prevailing Torque Fasteners
WARNING: This page is about a different car, the 2007 Pontiac Montana SV6, 2007 Chevrolet Uplander, and 2007 Buick Terraza. However, it is still accessible from the selected car via links, so may be relevant.
These fasteners accomplish the thread interface by the presence of a nylon material on the fastener threads.