The Buddha's death is seen as an illusion, he is living on in other planes of existence, and monks are therefore permitted to offer "new truths" based on his input. Mahayana also differs from Theravada in its concept of śūnyatā (that ultimately nothing has existence), and in its belief in bodhisattvas (enlightened people who vow to continue being reborn until all beings can be enlightened).
Who are enlightened people who vow to continue being reborn?
bodhisattvas