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Move will complete the transition to compatibility with AdWords enhanced campaigns.

To complete its path to compatibility with Google AdWords, Bing Ad has announced that explicit OS targeting will be retired in March of next year.
No longer will you have the option to target ads to a specific mobile operating systems as you are now.
Instead, what advertisers will see beginning in March 2015 is this:
This change rounds out Bing Ads’ adoption of Google’s enhanced campaigns. After showing some initial bravado when Google announced it was taking away tablet targeting from advertisers, Bing Ads backed down from that stance and decided it was better to join ‘em.
The effort to bring parity between the two platforms has been underway throughout this past year, with the thinking being that advertisers will be willing to spend more time in Bing Ads if they don’t have to learn how to navigate and manage campaigns in a completely different environment from the one they spend most of their time in, namely AdWords. In September, the ability to bid separately on tablet traffic was removed and, as on AdWords, desktop and tablet traffic is now combined. However, Bing Ads did give advertisers a concession here by providing a bid modifier on tablets, something that Google does not provide in AdWords.
At the same time, Bing Ads announced that App Extensions will roll out just as the explicit OS targeting goes away.

Source: http://searchengineland.com/bing-ads-retire-explicit-mobile-targeting-march-2015-207641