What happens in High Capacitance mode in Model 2450 SMU?

The SMU is a closed loop control device to maintain a steady force value on your DUT.

The capacitance of your cabling and DUT and the current sense resistor (internal to the SMU) form a pole. For certain combinations of current sense resistor and external capacitance, this can result in sufficient phase shift in the control loop to cause instability.

Enabling High C mode applies an internal capacitor across the current sense resistor to provide some phase lead to compensate for the phase lag created by capacitance of external DUT and thus restores stable control of the force value.

It has nothing to do with guard.

It has nothing to do with front panel vs. rear panel connections.

In vast majority of applications, High Capacitance mode is not needed. With 2450, you should not need to use this mode unless the load capacitance is greater than about 20nF.