An application must not be supplied directly from VBAT. This can interrupt the battery charging process causing unwanted behavior from the charger. Use either VOUT1, VOUT2, or VSYS to supply the application.
BUCK1 starts automatically and supplies the nRF5x host SoC with 1.8 V. BUCK1 is the I/O voltage for the system. BUCK2 starts automatically with 3 V output voltage for other application features.