A user clicks on a mobile ad for an iOS app. The ad points to a Google
  Analytics click server and the URL contains campaign information and the
  IDFA. Google Analytics collects the campaign information and the IDFA and
  redirects the user to the iTunes page for the app in the ad. The user
  subsequently installs the app from the iTunes page and this install will be
  matched to the ad campaign clicked by the user in the first step.