VDD is the main supply for the rest of the circuitry on the nRF9160 DK. It is regulated down from VSUPPLY by a buck regulator (U27).
Figure 1. VDD buck regulator and selection switch
You can set the VDD voltage to 1.8 V (default) or 3 V with SW9. Running 3 V General-Purpose Input/Output (GPIO) with heavy load may degrade the LTE RF performance. A third option for customizing VDD voltage level is applying a custom voltage level to VIO_REF (P20). For more information, see GPIO - General purpose input/output in the nRF9160 Objective Product Specification.
VDD powers most of the other circuits and will set the GPIO signal amplitude between nRF9160 and other circuits on the DK including connectors and PIN headers.