The build sheet is a set of instructions to the line workers telling them what options each car was to have installed in it. GM policy was that a build sheet is trash, and was to be disposed of as such. Human nature (protected by union rules) being what it is says that some workers didn't follow the rules and stashed the trash were QC inspectors and line supervisors didn't see them. If your car was heavily optioned you could find several build sheets in it in or around your optional equipment such as a console, or AM/FM Stereo radio, or deluxe interior instead of the usual cloth seats.
Build sheets have been found under the dash in the wiring with radios or stereo tape players, above the glove box if you got custom interior lighting, under seats for deluxe interiors, or perhaps inside a door panel or both as different workers with a copy of the build sheet working at different stations installed parts at different times. They have been found under the trunk above the gas tank if you got a larger tank than normal, between the inner fender and the cowl if there were special trim parts (such as the fender louvers for a Nova SS). They have been found above the headliner if you got a map reading light with your dome light (common in cop cars). Every RPO number could generate a build sheet.