The BQ25120A implements a simple voltage battery monitor which can be used to determine the depth of discharge of your battery. A high-level representation of the circuit can be seen on Figure 17 of the datasheet. Table 2 provides two different examples of typical VBATREG used on Li-Ion batteries to perform a simple 5-bar status indicator on your applications to allow user to see how much battery is left on the device. You can change your VBATREG on Battery Voltage Control Register (0x05) from 3.6V to 4.65V.
The specific battery used needs to be fully characterized to determine the threshold that will indicate the battery status. Then you can read the Voltage Based Battery Monitor Register (0x0A) to obtain the VMON read of your battery. This register read value represents a percentage of your battery value with 2% increments between 60% and 100% of your selected VBATREG. On your actual application you can use this percentage to estimate the stage of discharge of your battery. For additional information refer to Section 9.3.4 Voltage Based Battery Monitor of the datasheet.