Registers starting with a lowercase 'n' like nTGAIN and nTOFF are located in a space called Non-Volatile Shadow memory. This space is a volatile SRAM value that gets updated to the value of the relevant non-volatile memory data when the part is reset or through a recall command. While the part is powered this space can be modified by I2C as needed and behaves as a typical RAM.
Registers that don't start with the lowercase 'n' such as TGAIN and TOFF are also SRAM registers that are prepared at startup by values calculated by the firmware. If the firware looks at the shadow nNVCfg registers (non-volatile) and decides to use the nTGAIN and nTOFF values (via the enTGO bit), then TGAIN and TOFF will be prepared with the shadow values. If in this example the enTGO bit is not set, the firmware will look at the values that are saved in the ROM (firmware code) and use those values to setup the TGAIN and TOFF registers. In the case that default ROM values are used, this frees up the nTGAIN and nTOFF registers for use as general purpose non-volatile memory storage.
=============================================================Got this explanation from the BU AE===============================