Turkey – Customs Union and Preferential Origin
Most industrial products in free circulation in the European Union (EU) are also deemed to be in free circulation in Turkey, and vice-versa. Goods are in free circulation when all import formalities are completed, and Customs Duties are paid. You will find rates of Customs Duty that apply to imports of goods from Turkey in TARIC. You can use an ATR certificate as evidence of free circulation.
However, many agricultural, marine, coal and steel products must have proof of origin to qualify for preference.
You can find further information at European Commission - Turkey: Customs Unions and preferential arrangements.