The Multiplexer is a Logic Gate with 3 inputs and one output. Similar to the real-life version, the multiplexer's output toggles the logic state of either the 'left' or 'right' input, depending on the state of the 'bottom' input (as viewed from above, with the output as 'top').
The type of Multiplexer represented in Tekkit Classic is a 2-to-1 MUX.
Table assumes a view from above, with the output at the 'top'.
A: Logic Input A
B: Logic Input B
Z: Logic Output
|Bottom (S)||Left (B)||Right (A)||Top (Z)|