The TargetScore value determins the score a player must achieve to win the game. For example, the Desert War scenario uses TargetScore=6, as there are 6 objective cities in the game. If one team holds all 6 cities, then they win the game. By itself you can define the actual score a player must achieve (in the score list on the right of the interface), but coupled with python, this can be an extremely powerful scoring utility. You must have Victory=VICTORY_SCORE (or all victory conditions available) for this method to work.