531 is Anti-drive-off feature, central locking system
535 is Alarm 315 MHz
(
https://www.stuttcars.com/technical/option-codes/986-97-99/)
My 97 also has 531, basically it means you have central locking but no alarm system and no key remote. So I installed a similar aftermarket system to what you have there that provides both an alarm and remote central locking. The "sock sensor" (ha) detects sudden shocks and sets off the alarm if the car is bumped when it's armed.