Participation is voluntary, at the discretion of the game developer. To obtain the ratings for any piece of software, the developer completes a questionnaire, which is then evaluated by the Netherlands Institute for the Classification of Audiovisual Media (NICAM) and ratings are given. It's based on the Dutch Kijkwijzer system too.
There are two parts to the classification for any piece of software — a suggested minimum age and also up to six descriptions of content, such as the use of strong language, violence, etc.
PEGI has five age categories. In the countries of Finland and Portugal, local legislation conflicts with some of the ratings and they are changed accordingly.