Classification 1 provides the highest level of stability with both of those alarm and immobiliser functions put together. Category two gives only an immobiliser, which nevertheless prevents the car from starting off without having authorisation. Both of those are passively established and offer great security against theft. For more info https://medium.com/@trackerfit11/why-choosing-a-thatcham-tracker-strengthens-vehicle-security-6b79d8a9c256