DEC0 (pin 13) is an nRF9160-internal power management unit (PMU) output pin for an external decoupling capacitor.
An external capacitor improves the stability of the internal supply. The voltage level of DEC0 is approximately 2.2 V. The recommended capacitor value is provided in nRF9160 application component recommendations.
In the Printed Circuit Board (PCB) layout, place C7 close to the DEC0 pin and connect the ground capacitor to the same ground plane as nRF9160 VSS (pin 103).