Question about AI retreat mechanics

[Erenussocrates]

If you take a look at this guy's gameplay of campaign and watch it for just 2 minutes, you can see that he utilizes this to a great degree.

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But my question is, how does it work? When and under which circumstances do enemy AI's units give up charging and then just retreat? What are the factors that causes that? How do we know when AI would do that? I would definitely love a detailed guide about this doing extensive explanations.

PS: Warcraft 2 is a game older than 20 years, and at first glance it seems like a very simplistic game. But I believe there is still a heck load of things waiting to be discovered about it.