MapMixer is designed to give you more control over the way you launch and manage your game session in Unreal Tournament 3. It replaces the the game's Main Menu and let's you launch either a quick-offline match or a complete session where every map is specifically customised - or randomised. It's a whole new way to manage your UT3 experience.
Map List Editor - Changing Options for Specific Maps
As each map in your custom map list can be of a different gametype, MapMixer uses a Game Rules List (see below) to provide a base set of rules for the session, however, each map in your Custom Map List can also be customised, so that they have their own individual settings. Rules, Mutators and Bot Selection are available for each map. This screenshots shows how you can customise the bot selection per team for each map in the list.
The individual map settings are 'overrides' in the sense that they take priority over every other setting in MapMixer (except Player Caps). This is the reason why if you want to change the Goal Score for one or more maps in your Custom Map List, you need to add the rule using the 'Add' button, as it indicates that you want to override the base rule settings for that gametype.