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
The VDD voltage can be set 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. The VDD voltage level can also be customized by applying a custom voltage level to VIO_REF (P20). For more information, see GPIO - General purpose input/output in the nRF9160 Product Specification.
VDD powers most of the other circuits and sets the GPIO signal amplitude between the nRF9160 SiP and other circuits on the DK including connectors and PIN headers.