IMPC001 pinouts

I see a pin on IMPC001 named PSU_EN which has no description apart from psu enable on datasheet.
Does anyone know what this is used for and if it can be used as a GPIO line ?

It can’t be used for GPIO, no. This pin is connected to the modem power gate, so is on when the modem requires power; this can be used (very carefully) if a boost supply is needed to run the modem power.