The “FIT mode” was introduced in a late version of the Seeedstudio firmware, but I haven’t seen any official documentation. Nevertheless the idea here is to have the Nano automatically identify the type of waveform and adjust the controls to produce a usable display of the input signal. This is similar in function to the Auto push button found on most scopes. The function will track the input signal and continuously adjust trigger level, vertical sensitivity and horizontal sensitivity in small increments until a stable waveform can be displayed.