Advance
Regimental System
By organizing our armies into regiments tied to distinct geographical areas, we allow them to put down roots in local communities. Then, we need to raise additional battalions for these regiments, recruitment will be faster thanks to these regional ties.
Advance tree
The game files don’t say where this goes: it has no requires, so the game places it in the 
Absolutism tree itself.
Modifiers
Definitionin_game/common/advances/4_choices_mil.txt:334
336icon = regimental_system
337for = mil