nomadic_realm_split_effect

nomadic_realm_split_effect = {
	$ORIGINAL_REALM_HOLDER$ = {
		save_scope_as = root_scope
	}
	$NEW_RULER$ = {
		save_scope_as = new_ruler_scope
	}
	primary_title = {
		every_in_de_jure_hierarchy = {
			limit = {
				tier = tier_county
				holder.top_liege = scope:root_scope
			}
			add_to_list = de_jure_realm_counties_of_primary
			add_to_list = combined_counties_of_primary
		}
		every_in_de_jure_hierarchy = {
			limit = {
				tier = tier_county
				holder.top_liege != scope:root_scope
			}
			add_to_list = de_jure_realm_counties_of_primary_outside_realm
		}
		every_in_de_jure_hierarchy = {
			limit = {
				tier = tier_county
			}
			add_to_list = de_jure_realm_counties_of_primary_combined
		}
		every_in_de_facto_hierarchy = {
			limit = {
				tier = tier_county
				holder.top_liege ?= scope:root_scope
				NOT = { is_in_list = de_jure_realm_counties_of_primary }
			}
			add_to_list = de_facto_realm_counties_of_primary
			add_to_list = combined_counties_of_primary
		}
	}
	# Pick a suitable capital County, as far away from your capital as possible
	ordered_in_list = {
		list = de_jure_realm_counties_of_primary
		limit = {
			NOT = {
				this = scope:root_scope.capital_county
			}
		}
		order_by = {
			value = "title_province.squared_distance(scope:root_scope.capital_province)"
		}
		save_scope_as = new_capital
	}

	# Mark the capital duchy
	scope:new_ruler_scope = {
		scope:new_capital = {
			add_to_list = counties_given_to_sibling
		}
		scope:new_capital.duchy = {
			save_scope_as = capital_duchy
			add_to_list = duchies_given_to_sibling
			add_to_list = duchies_to_de_jure_shift
			if = {
				limit = {
					holder.top_liege = scope:root_scope
					holder.primary_title.tier < tier_kingdom
				}
				holder = {
					add_to_list = rulers_transferred
				}
			}
			every_in_de_jure_hierarchy = {
				limit = {
					is_in_list = combined_counties_of_primary
				}
				add_to_list = capital_duchy_counties
				if = {
					limit = {
						NOT = {
							is_in_list = counties_given_to_sibling
						}
					}
					add_to_list = counties_given_to_sibling
				}
				if = {
					limit = {
						holder.top_liege = scope:root_scope
						holder.primary_title.tier < tier_kingdom
					}
					holder = {
						add_to_list = rulers_transferred
					}
				}
			}
		}
		# Expand in a 'ring' first
		scope:new_capital.duchy = {
			every_title_to_title_neighboring_duchy = {
				limit = {
					any_in_de_jure_hierarchy = {
						OR = {
							is_in_list = de_jure_realm_counties_of_primary
							is_in_list = de_jure_realm_counties_of_primary_outside_realm
						}
					}
				}
				add_to_list = duchies_to_de_jure_shift
				if = {
					limit = {
						any_in_de_jure_hierarchy = {
							is_in_list = de_jure_realm_counties_of_primary
						}
					}
					add_to_list = duchies_given_to_sibling
					if = {
						limit = {
							holder ?= {
								top_liege = scope:root_scope
								primary_title.tier < tier_kingdom
								NOT = { is_in_list = rulers_transferred }

As written in game/common/scripted_effects/09_dlc_mpo_scripted_effects.txt.