From a 1200GS Technical Training manual.
Immobilizer code data structure.
The codes used during the handshake are generated from an Immobilizer Rolling Code table assigned to the VIN at the time of manufacturing. The BMS-K and corresponding ignition keys are encoded with this unique data making them an electronically mated pair for one particular motorcycle. The code table provides the codes for each key and a source for BMS-K to check the generated code for each key. Of the three keys supplied with a new motorcycle, each one is uniquely identified (key #, ie: 1, 2, or 3). Each key additionally then has a unique immobilizer code.
During the key on sequence, the BMS-K first identifies the key sending the code data and then confirms the transmitted immobilizer code for that key. There is the possibility of a maximum of ten keys per system possible. However they must be ordered specifically by the VIN, and activated with the GT1.