4D Provides many built in classes. Most of the built in classes can be utilized and are documented under the Class API Reference page of documentation site:
https://developer.4d.com/docs/Rx/en/API/overview.html

However, if the 4D Class store is inspected, it can be seen that there are some additional classes that appear but are not documented.

If the class is not documented it means that the class will not be useful to a developer due to limitations of the class. These undocumented classes are used by the application. The limitations of such classes are that it cannot be extended, the member functions are private, or there are some other limitations imposed on the classes.

As such, these undocumented classes will not be useful to developers are are not documented.