Game Mode Base is the parent class for creating Game Mode. A Game Mode class is used to define the rules of the game and specifies the default classes used for the creation of Pawn, PlayerController, GameStateBase, HUD, and other classes. To change these classes in Game Mode, click the Class Defaults button to display them on the Details panel, as shown in the following screenshot:

To specify the default Game Mode class of a project in the Level Editor, click Edit | Project Settings... Then, in the Project category, select the Maps & Modes option. Choose Game Mode in the Default GameMode property's drop-down, as shown in the following screenshot. In the Selected GameMode category, you can override some of the classes that are used by Default GameMode:

Each Level can have a different Game Mode. The Game Mode of a Level overrides Default GameMode of the project. To specify Game Mode of a Level, click the Settings button in the Level Editor and choose the World Settings option. Choose Game Mode in the GameMode Override property's drop-down, as shown in the following screenshot: