Dit is wat ik vind als ik Google:
How do you get Identification Document Number of ielts exam?
EDIT
Answered by The Community
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The Identification Document Number they are referring to is the number of the document you presented to them when you signed up for the test. which is usually either a passport number or an ID number.
Dus ik ga ervanuit dat het je IDnummer is en dus het nummer wat op je ID staat.
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