MDB don't have a client view. RequiresNew means that a new transaction should be started even if client calls a method within a transaction context.Since there is no client, RequiresNew doesn't make sense.
REQUIRES_NEW means the Client txn is suspended, and a new Txn is started. Since there is never a Client Txn in the case of an MDB, it makes no sense. MANDATORY,NEVER and the others also assume the existence of a well-defined client transaction scope.