DEC0 (pin 24) is an nRF9151-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.
In the PCB layout, place C7 close to the DEC0 pin and connect the capacitor ground to the same ground plane as nRF9151 GND.