सर्वेभ्य एव वक्त्रेभ्य: ससृजे सर्वदर्शन: ॥ ३९ ॥
pañcamaṁ vedam īśvaraḥ
sarvebhya eva vaktrebhyaḥ
Then he created the fifth Veda — the Purāṇas and the histories — from all his mouths, since he could see all the past, present and future.
There are histories of particular countries and nations and of the world, but the Purāṇas are the histories of the universe, not only in one millennium but in many kalpas. Brahmā has knowledge of those historical facts, and therefore all the Purāṇas are histories. As originally composed by Brahmā, they are part of the Vedas and are called the fifth Veda.