From reinforced tier servo and up, the right most button. (by default it looks like an arrow pointing to the right)
clicking this button will causes the setting to cycle trough the following modes, and the icon changes to indicate the modes:
1 nearest first (the default mode) An arrow pointing right.
2 Furthest first, An arrow pointing left
3 random, the five dots face of a d6.
4 round robin, arrows pointing at each other in a circle.