You have to understand how an ALT works in a normal situation and how one is controlled by an external reg. As I said above, there are two main ways of controlling.
In a non external reg situation, the ALT simply produces a continuous current into a battery. But as the Voltage rises, which is actually quite quickly, the current also quickly (proportionaly) drops. So the actual regulation is caused by the Battery load itself.
By using an external reg, the maximum output if the ALT is maintained no matter what the Voltage point of the Battery is. There are two ways of controlling the ALT in