The SPID is GM's factory option-list — a single adhesive label printed at the assembly plant with the VIN, body / paint / trim codes, engine / transmission / axle RPOs, and a comprehensive option block listing every RPO the car was ordered with. The label location moved over the run: 1982–1986 glove-box-or-console, 1987–1990 GTA console, 1991–1992 GTA rear hatch storage compartment (sometimes called the 'rear glovebox').
When the SPID is missing — peeled, lost in a re-trim, faded by sun on a convertible — the door VIN/data plate is the next-best cross-validation source. plenum decodes the SPID when present and falls back to door-plate cross-checks when the SPID is gone.