GM uses Regular Production Option codes — three-character alphanumeric identifiers (later four) that name every option a car can be ordered with, from mirror types to engine families. RPO codes are printed on the Service Parts Identification label, the SPID, which is a sticker affixed somewhere inside the car. Three-character codes (B2L, WS6, Y84) coexist with four-character codes (AAB1, AU3K). The four-character codes are typically option sub-variants that came online when the three-character namespace got crowded, and they appear more often on later cars.
Some codes are reused across model years with different meanings — Y82 is the Formula merchandise package most years but specifically denotes the 1989 20th Anniversary Turbo Trans Am. Some codes mean different things on different platforms. Plenum's vocabulary is keyed by both year and platform so this works correctly.