There was once a chat with Aaron about all this but unfortunately the thread has gone in the Last month or so.
Exploration is the state until the AI meets another race. Then it will nearly never return to it IIRC (it will return to it for example if you cut all warp points between the AI and all other races).
In the end it is very hard to predict to what state the AI will switch. Because of this I put all the build queues in only 3 categories.
One is “Not connected” and it is obvious that you need to build and research in quite a different way when you are locked in one system.
The “exploration” phase is important because it will be the first state the AI is in. Don’t overload this state with build orders as it can strip the AI off all resources due to maintenance and no building will be finished.
I put all the rest in one state because the AI has no real concept of attack and defense. So it is of no use if you build more defense units or attack units, the AI will need both. It was planned that the AI would build up for an attack but as it seems it never really worked.