On my IC-7300 I normally leave it off for max sensitivity, but during a contest weekend when strong signals flood the bands, I will cut it on. I'm guessing it works the same for the R8600
The IP Plus function improves the Intermodulation Distortion (IMD) quality via DSP. This function optimizes the Analog/Digital Converter (ADC) against the distortion when you receive a strong input signal. It also improves the Third-order Intercept Point (IP3) while minimizing the reduction of the receive sensitivity.
Select ON to prioritize the IP quality, and select OFF to prioritize the receive sensitivity.