These values explain the current leakage and the capacitive coupling between the signal and the control terminals. Parasitic resistances and capacitances between the signal terminals IN and OUT and the switch control terminals explain current leakage and noise coupling into the signal path. When a switch closes, R m is the resistance in series with the signal path. R m can vary significantly between mechanical relays, where R m is a few milliohms, and solid state switches where R m can be several hundreds ohms. In the case of an electromechanical relay, V m represents the thermal EMF voltage that the switch contact generates and which appears in series with a signal.
Thermal EMFs result when two different metals make contact and coexist in a temperature gradient.
Even if contacts are of the same material, they can still generate a small EMF because their composition always differs slightly. In addition, temperature gradients can also exist between the two sides of a contact inside the same switch enclosure. For solid-state relays and FET switches, V m is mostly due to voltage offset in the semiconductor material that makes up the switch. V m can range from a few microvolts for mechanical relays to several millivolts for solid-state relays. If you think that these parasitic elements remain constant and you can therefore compensate for them when calibrating your system, bear in mind that some of these values change with time and with changing physical environmental parameters, such as humidity and temperature.