Could anyone tell me what is the usage of the circuit within the red frame
It is a voltage divider. The whole circuit is a differential amplifier with 2 levels of amplification (S1). The dicvider is made of precision resistors with extra potentiometer P1 to calibrate the input division, if you short circuit the inputs and add a common mode voltage the output should be zero (trimming the P1).
The capacitors are for matching the impedance of the probe. RC for each section must be equal. It is the same as calibrating osciloscope probe and caps are for the same purpose.