struggle_cb_title_transfer

struggle_cb_title_transfer = {
	save_scope_as = target_title # For the vassalization trigger.

	# Compile a list of all counties in the kingdom we will take from the defender.
	every_in_de_jure_hierarchy = {
		continue = {
			tier > tier_county
		}
		limit = {
			tier = tier_county

			# Current holder must be a (sub)vassal of the defender (no poaching titles from 3rd parties!)
			holder ?= {
				OR = {
					this = scope:defender
					target_is_liege_or_above = scope:defender
				}
			}
		}
		add_to_list = titles_taken
	}

	every_in_list = {
		list = target_titles
		limit = {
			tier = tier_county
			holder ?= {
				highest_held_title_tier < scope:attacker.highest_held_title_tier
				OR = {
					this = scope:defender
					target_is_liege_or_above = scope:defender
				}
				any_sub_realm_county = {
					count = all
					is_in_list = titles_taken
				}
			}
		}
		holder = {
			if = {
				limit = {
					NOT = { is_in_list = potential_vassals }
				}
				add_to_list = potential_vassals
			}
		}
	}

	ordered_in_list = {
		list = potential_vassals
		order_by = highest_held_title_tier
		max = 99
		check_range_bounds = no

		if = {
			# If a liege has already been added to the vassalization list, skip us (we will remain our liege's vassal.)
			limit = {
				save_temporary_scope_as = this_vassal
				any_in_list = {
					list = vassals_taken
					target_is_vassal_or_below = scope:this_vassal
				}
			}
		}
		else = {
			# Flag this character to be vassalized instead of usurped.
			add_to_list = vassals_taken

			# Remove the counties this vassal holds from the seized title list (the vassal gets to keep their land when switching allegiance!)
			every_held_county = {
				remove_from_list = titles_taken
			}
			every_sub_realm_county = {
				remove_from_list = titles_taken
			}
		}
	}
}

As written in game/common/scripted_effects/00_war_effects.txt.