Before answering this question, it is necessary to explain the Buddhist doctrine of Panca-yana (Five Vehicles). Yana means a vehicle which can carry passengers to their destinations, far or near. It is used here as a metaphor of the Buddha’s teachings. The doctrine of Panca-sila (Five pre-cepts: abstaining from taking life from sentient beings, taking things not given, sexual misconduct, false speech, the use of intoxicants) enables the practitioners to be reborn in the human world, so it is called Manssa-yana (human-vehicle). The doctrine of Dasa-kusalani (Ten meritorious acts-avoidance of the ten evils: 1. killing, 2. Taking things not given, 3. Sexual misconduct, 4. irresponsible speech, 5. Speech inciting discord, 6. Harsh speech, 7. Licentious talk, 8. Covetousness, 9. Anger and 10. Heterodoxy, i.e. denial of the doctrine of cause-effect) enables the practitioners to be reborn in heaven, so it is called Deva-yana (heavenly vehicle). The doctrine of Four Noble Truths (ariya-sacca, namely, the truth of Suffering, of the Origin of suffering, of the Extinction of suffering, and of the Eight-fold Path leading to the extinction of suffering) enables the people to discard wrong views such as ego-illusion, eternity-belief, annihilation-belief, etc., and defiled thought such as greed, hatred and delusion, finally, to attain nibbana (enlightenment). So it is called savaka-yana (voice-hearer vehicle). Voice means the Buddha’s teachings. Those who hear Buddha’s teachings, realize the Four Noble Truths and are free from sufferings are called savaka. Before the advent of the Buddha-Dhamma, people who independently came to the realization of the Theory of Paticcamupada (Dependent Origination) and obtained emancipation, but were unable to speak out their self-enlightened truth were called Pacceka-Buddha (Independently Enlightened One). That is why the Dhamma of Dependent Origination is called the vehicle of self-enlighenment. The doctrine of Six Perfections enables the practitioners to act in the way of Bodhisatta, to undergo the countless difficulties and hardships in cycles of birth and death, and to finally attain Buddhahood. So this teaching is called the vehicle of Bodhisatta. (From Essentials of Buddhism: Questions and Answers)