DM’s running Undefeated sessions shall award experience using the standard DMG rules located on page 38 of that book.
At the beginning of a session, players shall determine who their primary character will be. This is the character they will play during that session and he or she shall receive 100% of the session’s experience.
Players will also choose one of their other characters at the beginning of the session to be their Secondary Character. This character shall receive 75% of their primary character’s experience.