ARP = Average Rank PositionATRP = Average Total Rank Position
The difference is that the ARP is an average of only the pins that actually have a result (position 20 or less). ATRP averages all the pins, even when a business is not found (20+).
This difference helps you compare if a business is ranking well across a wide area, or if they are ranking well in only a small area within the search grid.