common_character_validity_trigger

common_character_validity_trigger = {
	custom_description = {
		text = "FACTION_CANT_JOIN_CREATE_IMPRISONED"
		trigger_if = {
			limit = {
				joined_faction ?= {
					faction_is_at_war = yes
				}
			}
		}
		trigger_else = {
			is_imprisoned = no
		}
	}

	OR = {
		$FACTION_TARGET$ = liege
		any_character_active_contract = {
			task_contract_target = $FACTION_TARGET$
		}
	}
	highest_held_title_tier > tier_barony

	#Not blocked through events
	custom_description = {
		text = character_blocked_from_joining
		NOT = {
			has_character_flag = joining_faction_block
		}
	}
	
	# You cannot start a faction against your bloc members
	trigger_if = {
		limit = {
			government_is_japanese_trigger = yes
			$FACTION_TARGET$ = { government_is_japanese_trigger = yes }
			exists = house.house_confederation
			exists = $FACTION_TARGET$.house.house_confederation
		}
		custom_tooltip = {
			text = house_confederation_faction_blocker_tt
			house.house_confederation != $FACTION_TARGET$.house.house_confederation
		}
	}
	
	NOT = { has_relation_blood_brother = $FACTION_TARGET$ }
}

As written in game/common/scripted_triggers/00_faction_triggers.txt.