When sharing previews with clients, you might want the first impression of the creative to be from a mobile device. To make that happen, choose the respective device from the icon during the creative build. Or vice versa, if you want the creative to be shown in a desktop device first.
Initially, the platform chooses the default preview device to display the ad on, based on the creative size chosen.
When the ad type chosen is interstitial or interscroller, the default is always mobile. For skin, in-stream (VAST/VPAID) and lightbox the default is always desktop.
In two instances, the creative by default is desktop:
When the creative is a wide leaderboard
width > 320 ja
width > height * 2
When the creative is a skyscraper
height > width * 2
In all other instances the default preview is shown in a mobile device.
When you make the choice there, that will be the default device for the preview site as well.