APP stands for Alternate Price Projection (Robert C. Miner's invention). I add to every APP I (for impulsive) or C (for corrective). APP is a measure of the previous move in the same direction, some people call it colloquially AB=CD extensions or something like that.
So, I-APP-0.500 means 0.50 (ie. 50%) of the size of the previous impulsive move in the same direction as the current one. That's where the point P comes (this method is known as the trident strategy by Charles Lindsay).
I hope you have it all clearer now. :-)