Per Capita vs. Per Stirpes
Imagine a parent with three children: A, B, and C. A is alive. B has died, leaving two children. C has died, leaving one child. Under per stirpes, A takes 1/3, B's two children split B's 1/3, and C's child takes C's 1/3. Under strict per capita, all four (A, B's two children, and C's child) take an equal 1/4.
Arizona's Default: Per Capita at Each Generation
Arizona's default under A.R.S. 14-2709 is "per capita at each generation." Each generation that has a living taker is treated equally, then the remaining shares drop down. A trust or will can override this by specifying per stirpes if the family wants the strict family-branch approach.