What do the difficulty levels actually do (DE and RoR)? Can't find anything referring to them in .ai/.per guides, or scenario editor

How does the difficulty level affect the computer's behaviour? I have looked at the AI guides on http://aoe.heavengames.com/siege/AIandPER/PerFilesPhill.shtml and nothing refers to difficulty level. The .ai files are essentially a build order, and the .per file is a bunch of adjustable values that change the personality.

So I'm assuming that the difficulty level affects things that are hidden in the engine somewhere.

So: When I play a scenario and select the difficulty level, how does that actually affect what the computer player actually does in terms of its actions? Does it affect how quickly the AI gives commands or waits between training new units?