Openings classification in ChessBase refers to the identification of the openings variation in the openings key.
This is how it works:
The program plays through each game backwards and compares every position it encounters with all the positions contained in the classification positions of the openings key. As soon as a match is encountered it knows under which key it must classify the game. The reason for playing through the game backwards is that we are interested in the deepest position, i.e. the one that identifies the most detailed variation.
When you install a new key (Tools – Classification) in the list window or click the Openings tab) all the games of the database are classified into the key in the same way.
Classifying the current game
In the board window click Report – Openings classification (or press Ctrl-Alt-C).