The nIO_PULLUP pin is a pin that sets the state of user I/O pins (including Dual-Purpose pins) after power-up, before configuration begins, and during configuration.

・ nIO_PULLUP = High (Directly connected to VCC. Check the handbook of each device for details.)
⇒The internal Weak Pull-Up resistor is disabled, and the device enters a Tri-State state without Weak Pull-Up.
・ nIO_PULLUP = Low (directly connected to GND)
⇒The internal Weak Pull-Up resistor is enabled, and the state becomes Tri-State with Weak Pull-Up.

For details, search for "nIO_PULLUP" in the Device Handbook of each device and refer to it.