In the games list you see the following tabs:

 

 

These are classification systems in which the games are automatically sorted by the program.

 

Openings keys are defined by positions. The deepest classification position decides into which key the game goes.

 

Theme keys are defined by the search mask. All games that match the criteria are sorted into the key associated with the definition.

 

As part of the ChessBase package you get pre-defined keys for large and small databases. If you are technically minded you can modify them or develop your own keys and classification systems. This is especially rewarding in the area of theme keys.