From Vivid Unit
This is the approved revision of this page, as well as being the most recent.

Vivid Unit’s 40-pin GPIO header has compatible layout with Raspberry Pi. All I/O pins are at 3.3V level.

Although RK3399 provides GPIOs in 3.0V, Vivid Unit adds level shifters to all I/O pins to make them 3.3V. Most I/O pins are using a MOSFET for level shifting, while some I/O pins are using a level shifting chip for better speed.