Trade routes
Trade routes between two markets which do not share a common land border must be done overseas and will necessitate a shipping lane. Land adjacency is determined from where the two market capitals are located. The convoy cost is influenced by the number of sea nodes, quantity of goods and any goods-specific modifier. The effectiveness affects the trade route competitiveness and by extension the quantity of goods shipped. It will use the two closest ports in the respective market capitals region. If either country lacks ports no overseas trade routes can be established.
Supply routes
Supply routes are required when a general is sent to a front that is not reachable by land. It will use a friendly port connected by land to the generals headquarters and trace to the closest friendly port reachable from the front. The convoy cost is based on the number of sea nodes, battalions supplied and any general traits. Low effectiveness reduces supply status of the general and his troops. If a front is landlocked no generals can be sent there.
Tariffs
Tariffs are the means where a national government extends its influence as an intermediary in the trade between national markets, if not for the means of protecting its national interests, to at the very least ensure it gets its fair share of the profits that such entails. Tariffs are set on both exports and imports leaving the national economy because yes the government is interested in its fair share and if it cannot get the revenue by means of a consumption tax it will find other means. The ratio of this tariff level is dependent on the trade policy set. A more mercantilist trade policy would seek to ensure exports exceed imports so tariffs on exports will undoubtedly be lower. Protectionism is equal in its ratio as it seeks to shelter the domestic economy from booming or busting on either side of the equation. Free Trade.
While the laws set the tariff ratio of import/exports these can be customized further in the budget screen by setting their tax levels. Tax levels don’t just bring in revenues but offer incentives to your economic actors, your pops. Lower tariffs encourage trade while higher tariffs will hinder their efficiency because well if the nation is getting a bigger cut.