Mike,
VSS stands for Vehicle Speed Sensor. The SACC unit has a knob that you turn to increase or decrease the amount of assist provided by the EPAS. Unisteer works the same way (if you purchase the variable assist module). The EZ Electric system utilizes a VSS, which decreases the level of assist automatically, as vehicle speed increases. By the time you get to highway speeds, the EPAS assist decreases to zero. This is a nice feature but increases the price significantly.