The UDID is a long string of alphanumeric characters which will appear in place of the serial number.

UDID是出现在序列号内的一长串文字数字式字符。

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Here you must enter your email address and UDID as the first step in the AD Hoc distribution process. The UDID, or Unique Data Item Description, is available from within iTunes.

Ad Hoc distribution第一步是必须输入你的电邮地址和UDID, UDID即唯一数据条目描述，在iTunes里可以获得。

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Another way to identify devices without using a UDID is device fingerprinting.

FORBES: Apple Will Make It Harder For Apps To Track iPhone/iPad Users

She notes that programming hooks in iOS 6 will provide an alternative to UDID for device-specific data, and that apps will eventually be forbidden from using the older identifiers altogether.

ENGADGET: Apple denies giving FBI any iOS device UDIDs, raises questions over AntiSec claims

First, you need your UDID.

FORBES: iPhone And iPad UDID Data Not Stolen From FBI