How are modrons chosen to be promoted?

I am about to run an encounter of the week featuring modrons that includes combat encounters with monodrones and duodrones.

The stats for modrons state:

Cogs of the Great Machine. If a modron is destroyed, its remains disintegrate. A replacement from the next lowest rank then transforms in a flash of light, gaining the physical form of its new rank. The promoted modron is replaced by one of its underlings in the same manner, all the way to the lowest levels of the hierarchy.

Canonically, what is the mechanism that decides modron promotion? Would the physically closest modron (in the combat) be promoted? Would it be random? Would the promoted modron be on the same plane? Would it be Mechanus?