The piece features a two-column chart, one side with the letter "A" at the top and the other with the letters "M," "P" and "Z." While he hasn't spoken out about the tattoo yet, many are speculating that the "A" is for Angelina, while the three other letters represent Maddox, Pax and Zahara.
It's clear there are additional letters after the "Z," so we can only assume Shiloh, Knox and Vivienne have some representation on there as well.