Is Turkey in “the” EU Customs Union or in a bilateral customs union with the EU?
Turkey is in a bilateral customs union with the European Union, titled the “European Union–Turkey Customs Union”. This bilateral customs union arrangement notably does not cover agricultural services.
The Customs Union entered into force on 31 December 1995. It covers all industrial goods but does not address agriculture (except processed agricultural products), services or public procurement. Bilateral trade concessions apply to agricultural as well as coal and steel products.
More information here.
Turkey is not in the EU Customs Union which consists of solely EU member states.