2.2.3 Power-on Reset (POR)
Information regarding POR reset and other reset sources can be found in Reset Register Status.
When the user sets the voltage on the slider to POR level, the POR will take control and hold the device in reset until the voltage rises again above the threshold.
The POR LED is connected to the board controller. Since ATtiny3217 is in POR reset when this LED should be ON, it cannot be controlled by ATtiny3217 itself.