The **second slot** on the original **PS Vita (PCH-1000 / PCH-1100)** is exclusively for **PlayStation Vita Memory Cards** (proprietary format).
Replaces Game Card slot with MicroSD (up to 2 TB tested). **Must have HENkaku/Enso (3.60/3.65/3.68/3.70/3.73/3.74)**. Game Card slot becomes unusable for physical games while inserted. Best value: 256–512 GB MicroSD ($20–$40) vs $100+ for 64 GB Vita Card.
TL;DR: Slot 2 = Vita Memory Card (PCH-1000 only). Slim has no slot 2 (uses internal 1 GB + sd2vita mod).