Supporting documents are documents that you need to take with you in your trip to Turkey, like another country´s visa in your passport, proof of return flights etc.
You will show these supporting documents to the Turkish Immigration officers on arrival in Turkey.
Supporting docs could include:
1) a Schengen Visa
2) a visa from any of the OECD countries
3) Return flight tickets
4) Travel Insurance Papers
5) Bank Statements or other proof of funds
Visit our website for more information: https://turkishvisa.com/