APP is Accelerator Pedal Position sensor, mounted on the pedal. TAC is Throttle Actuator Control module. On the truck engines when they switched to the GenIV they quit using the TAC module, this is on new body style 2007 trucks, they also made the old body style truck in 2007 that has the GenIII and uses a TAC module.

 

APP is another name for the throttle pedal. Of course the pigtail connects the TAC and APP. Seems that the TAC plugs are all the same but the APP end will be different from 2005 on. As near as I can figure out the whole thing changed in 2005 and again half way thru 2007.

 

The TAC modules went away with the Gen IV motors, starting the the ls2 in 2005, but the 5.3's didn't switch over until like 09, if you really want a gen IV you have to be careful with truck pulls in those few years. Also The Gen IV motors aren't quite so plug and play with the cruise control, since the switches hook up to the BCM and then come to the PCM over GMLAN.