A few things affect fatigue: (1) Obviously the preferences setting. You can switch it on or off. (2) Movement rates. Units tire much more quickly with slower movement rates. Note that these are the global movement rates set in descr_battle_map_movement_modifiers.txt, not the unit movement rates set in DMB. In XGM I recommend that users switch off the slower movement rates if they play with fatigue on. (3) Units recover from fatigue more quickly if they have the hardy or very_hardy attributes in EDU (this may not help the AI because often it won't stand still even for a few seconds to benefit from faster recovery). (4) I think heat penalties in EDU increase fatigue in hot climates. (5) I don't think unit movement speeds in DMB have an effect, except in so far as it takes slow moving units longer to get anywhere, so they are more tired when they arrive.