When you use the CS port vs. the High Power Contact port, the difference physically, as commented above, is that there is a 10k Ohm resistor inline with the CS port.
What this does is limits current flow.
When you use frequencies that are over 10kHz on either port, you won't usually feel anything as there is no resonance with your nervous system.
The reason you feel the lower frequencies is that it resonates with your nerves and causes the muscles to fire.
Typically use the CS port if you are running low frequencies and felt too much even with the contact settings in effect to reduce the likely hood of feeling too much (Reduce Amp, Amp Ramp, etc.).
Now for the other main difference between the two.
The High Power Contact port allows higher harmonic frequencies to be passed into the body. The 10k Ohm resistor that limits current in the CS port also filters these very high harmonics out.
The CS port was initially created to limit current to provide a better Colloidal Silver creation process. The side effect is that it also provides a very safe contact session as you can not get too much current in this mode.
However, some devices like PEMF would not benefit from limited current, so the High Power contact port was then added in the Boost 3.0
That pretty much covers the difference between the two.
For more details, please check the link: