In other words, a "legal move" as put in the English version of the BG rules refers to a move that is allowed by the BK rules, including a move that is against the MDU rule.
(hide) If you want to play a game with an opponent of a similar level, you can define a required BKR range for a new game invitation. Then nobody with a BKR outside this range will be able to see/accept it. (Katechka) (show all tips)