VCell is measured via the CELLX pin. The LSB of the CELLX measurement is shown on page 7 of the datasheet (19-100318; Rev 0). Moreover, the IC uses 16 bit registers and the voltage measurements are unsigned. You can use the equation below to read back the correct voltage:
VCell = 78.125uVxVCELL[15:0]; where VCELL[15:0] represents the decimal value of register 09h.
For example, if VCELL[15:0]=0xD203 the equivalent decimal value is 53763, so VCell=78.125uV x 53763 = 4.2002V.
For multi-cell applications, the divider circuit on CELLX must be selected appropriately to read back the correct per cell voltage. The recommended divider circuit is shown in Figure 5 of the datasheet (19-100318; Rev 0).