Output isolation describes how well the input and output are isolated from each other. The higher the isolation, the better. The lower the isolation becomes, the more of a chance small signals can be lost going through the contacts. If one is switching small signals (digital signals or smaller), the reed sensor is the best option with greater than 1 x 1012 Ohms isolation, with Hall sensors having the worst isolation at 1 x 107 Ohms, and electromechanical sensors a little better at 1 x 108 Ohms.