In the datasheet it says it is between 4.05 V to 4.6 V but where and how is it used?
Battery Regulaiton (BatReg) is the voltage to which the MAX20303 will charge the battery. So if BatReg is set to 4.20V, the BAT pin will attempt to drive itself to 4.20V when the charger is active. This action charges the battery, but, the voltage measured at BAT will be the battery voltage, not BatReg. When the real battery voltage matches the BatReg value, the charger considers the battery charged and will stop charging or enter maintain charge mode per the charger configuration settings.