The registers are reset if the ACTIVE supply is cut off. It depends on how POWER CONTROL AND TRICKLE CHARGER block is configured. This determines what the active supply will be.
Either the VCC or the VBAT can be used to generate VCC internal as shown in the block diagram below. This controls the POR block.
Users may also use the data retention to preserve data. Data can be preserved in this mode as long as the active supply is present. To enter the Data Retention mode, write "1" to DATA_RET in the RTC_config1(03h) register. To exit the Data Retention mode, write "0" to DATA_RET in the RTC_config1(03h) register. Active supply can be changed using information given in Table 1 on the datasheet which controls the Power Control and Trickle charger block shown below.