From this screen you can select the classes of your character.
Use which class for saving throws: This is used mainly for AD&D 2nd edition where you must select which class RPA will use to calculate the saving throws.
Do nothing, i will manage XPs and levels myself: if selected, RPA will not automatically input the experience points of your classes.
Adjust the level according to the XPs: if selected, RPA will overwrite your class level based on the number of experience points you enter. If you enter 2000xps and that corresponds to a 2nd level fighter, then the level will be adjusted to 2 automatically.
Ajust XPs according to the level: if selected, RPA will overwrite your experience points based on the level of your character. If you have a 2nd level character, then RPA will overwrite the experience field with the minimum experience points needed for that class.