[RightDeve]

There are scenarios designed exclusively for VS AI play, there are those exclusively for PBeM, and then there is a few that can be played for both VS AI and PBeM.
Also you can read the instruction for which sides to play on the scenario description. If you try to play deviating from what has been designed, more often than not the game will be unbalanced. This is especially true for those designed VS AI, since the AI side is mostly given a good advantage (more units, unbelievable unit dispositions, etc) by the scenario designer to give a decent challenge for the human player, taking into account that the AI is less able to devise a good plan unlike human opponents.