Stock speed sensor is a reed switch for mechanical dashes. It's a photo coupler in digital dashes. Both are located integral to the speedometer. Both generate 4 pulse per rev of the cable. Loss of it will cause a code. They can be replaced by any device that provides a compatible 5 volt pulse train. From what I gathered by reading the data sheet the output of that electronic speedo can be calibrated to do that.
The signals' mnemonic is SPD. SPD is used by:
1) Cruise control. It's the main input for this system.
2) TEMS, for preventing anti-dive when the brake pedal is pressed unless the car is going over appx 40 mph, and to enable high speed firm when at or above appx 80 mph.
3) TCCS:
a) For determining decel fuel cut and resumuption.
b) For air fuel ratio correction during engine transitions.
c) For turning on the MIL in the presence of certain codes, including a loss of itself.
d) For idle speed control.
e) For some automatic transmission functions.
SPD has varying degrees of impact depending on the function. For example in air fuel correction during transitions it's minor while impact on idle speed control is more pronounced.
The ABS system is autonomous, therefore it doesn't use SPD.
