datasheet 37 page "6V to 300V High Side Power, Charge and Energy Monitor"
What is the difference between FGND and GND symbol connect to Q3(2N3904)'s EMITTER?
FGND is the floating GND node and is ~5V below Vin. This value is set by the Zener diode (Z1). This allows the LTC2946 to be powered by 5V while dropping the remaining voltage through the transistors below. Something else to look into, as a replacement for the transistors and the optocouplers is the ADM3260.
Thank you and best regards,