expedition_cb_title_transfer

expedition_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 = {
			OR = {
				tier = tier_county
				tier = tier_duchy
			}

			# 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
	}

	# If we can take the current holder of a title as a vassal instead of siezing their land directly, do so.
	every_in_de_jure_hierarchy = {
		limit = {
			tier >= tier_county
			trigger_if = {
				limit = { $RELIGIOUS_WAR$ = yes }
				religious_cb_holder_under_target_can_be_vassalized = yes
			}
			trigger_else = {
				conquest_cb_holder_under_target_can_be_vassalized = yes
			}
		}

		# Mark this vassal as being eligible to transfer from the defender to the attacker.
		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_sub_realm_county = {
				remove_from_list = titles_taken
			}
		}
	}
}

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