According to Wikipedia. UDID is an acronym for Unique Device ID. This UDID is actually a feature of Apple iDevices.
You can walk through this automated process to get it or follow the guide below.
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Connect your iPhone, iPad or iPod Touch with the USB cable.
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Start iTunes on your Windows or Mac.
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Now select your iPhone or iPod under the Devices menu which is on the left side.
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Click your Device Name like “Samrat iPod” and open the Summary Tab.
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In the Summary Tab you will find Serial Number. Click on “Serial Number”.
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After clicking ‘Serial Number’ it changes to “Identifier (UDID)” and displays the UDID.
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Then copy (Ctrl + C on Windows and Command + C on Mac) the 40-digit UDID.
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Finally, paste into an email to: The iOS developer that contacted you!