VSS and Transmission Output speed

[Bruce Plecan]

On a 8625 pcm, there are two connections, one labeled VSS, which is vehicle
speed sensor, and a second one labeled transmission output speed,   they
both feed from the Vehicle Speed Signal Bufffer to the PCM.  Can anyone
splain this to me?.  Is this just a way of checking for transmission
"slippage".
Anyone happen to know what this 5v signal should be?.  ie square, 4,000ppm?.
Thanks
Bruce