develop_celestial_tributaries

develop_celestial_tributaries = {
	if = {
		limit = {
			subject_contract_has_flag = celestial_investiture_trade
			capital_county.development_level < overlord.capital_county.development_level
			subject_standing > 10
		}
		save_scope_value_as = {
			name = tributary_capital_development_growth
			value = {
				add = { # ratio of how close they are in development
					add = capital_county.development_level
					divide = overlord.capital_county.development_level
				}
				subtract = 1
				multiply = -1
				multiply = { # ratio of subject standing, with a minimum value to make at least something happen
					add = subject_standing
					divide = 100
					min = 0.2
				}
				multiply = celestial_investiture_trade_bonus_max
				add = { # to make it feel a bit more dynamic
					fixed_range = {
						min = -5
						max = 5
					}
				}
			}
		}
		if = {
			limit = { scope:tributary_capital_development_growth >= 1 }
			send_interface_message = {
				type = msg_tributary_capital_province_development_growth
				title = msg_tributary_capital_province_development_growth_title
				desc = msg_tributary_capital_province_development_growth_desc

				left_icon = root
				right_icon = overlord

				capital_county = {
					change_development_progress_with_overflow = scope:tributary_capital_development_growth
				}
			}
		}
	}
}

As written in game/common/scripted_effects/10_dlc_tgp_scripted_effects.txt.