The common-mode or differential-mode designations assume two conductors, one in-phase, the other anti-phase, but a static circuit is different because the "conductor" is concentrated in thin lines during discharge, but distributed (and delayed) during the return, so it's as if the signal is just "mono" and neither common-mode, or differential-mode.
The common-mode choke is used to protect against common-mode noise which comes in both lines with the same polarity and amplitude, and can then oppose each other when trying to energize the magnetics of common-mode choke.
I understand that, in addition to common-mode noise, there is another kind of noise called differential-mode noise, which I assume goes in one line and back out the other, with equal and opposite polarity, which I would assume would be similar to the signal in one twisted-pair of an ethernet cable. But that's a desired signal. There are obviously other differential-mode energies that can be thrust upon a pair of conductors that are not-desired, where the energy goes out one wire and comes back in another wire (though I'm not sure what this typically is).
Static electricity is also different because the voltage is so much higher, creating a circumstance where it comes in only one of the conductors, so it is therefore neither common-mode nor differential-mode, but some kind of half-differential mode (or mono mode). Each line having its own choke (in my mind) would be a way to protect against this. But I hadn't heard about a third mode, where the return for the circuit would be outside of the device and not coming back on any conductor. Would you please tell me the name of this "third" mode, and a little bit about it? Thanks.