A/D stands for Analog to Digital. The means that the 0 to 5 volt "analog" value is converted to a "digital" number that the PCM can use for processing. The A/D Converter in the PCM is a 10 bit converter which means that it has 1024 (2 to the 10th power) steps on the scale. Since the analog voltage scale is 0 to 5 volts, 0 volts = 0 A/D Counts (or "ticks" as Ford refers to them) and 5 volts = 1023 A/D Counts.
As to the "shutoff" error with excessive APP, this is because the maximum allowable APP is 922 A/D Counts, or roughly 4.5 volts. Anything about that is considered a pedal fault and the ECM instantly drops the engine to idle to prevent an accident or a runaway engine. At that point, you MUST drop the APP voltage back to an idle value and reset the IVS before the throttle will function again. This value can be raised in the parameters, but I've never found any reason to unless the throttle pedal has been modified.
Hope this helps.
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