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

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