Context: According to Mahayana tradition, the Mahayana sutras were transmitted in secret, came from other Buddhas or Bodhisattvas, or were preserved in non-human worlds because human beings at the time could not understand them:

Question: What sutras were transmitted in secret?
Answer: Mahayana

Question: What sutras could have been preserved in non-human worlds?
Answer: Mahayana

Question: What tradition says that sutras might have come from other Buddhas or Bodhisattvas?
Answer: Mahayana tradition