U1 pin 5 is connected to one of the internal reference voltage divider taps and C4 filters this point to reduce effects of noise on the power supply. Test point TP1 provides a reference voltage for calibration purposes that can be compared to TP2 (adjust ON Adj so that TP2=TP1) and TP3 (adjust OFF Adj so that TP3=0.5 x TP1).