As spike, my understanding is generation 2 stereos that talk through can bus are learnt by the computer and stored, if they are removed or disconnected from power when reconnected the car checks the unit and knows it so it works. Pre can bus stereos do not use can for communication and the code is needed every time the power is disconnected for more than a certain time. That is the way I always expected it to be. The jump lead or power pack method can be used on both types. Although if gen 1 I think I would get the code regardless as you are only a flat battery or bad mechanic away from non functioning stereo.