3-port isolation is the technical standard for all Knick isolators and transmitters, i.e. the input, output and power supply circuit are galvanically isolated from each other. With 3-port isolators, input and output circuits can be set to any user-defined potential as long as the allowable working voltages are observed.
Some manufacturers who do not supply isolators with 3-port isolation as a standard differentiate between input isolators, output isolators, and 3-port isolators. Again, this differentiation is not necessary to pay close attention to when selecting Knick products, as 3-port isolation is the technical standard here.