What is Ad Selection Mode?

The "Ad Selection" option can be set on campaign level (under Details > Information > Campaign Settings) to determine which advert should be selected. You can select whether the delivery should be "Even" or "Best". If you select "Even" all ads are shown with even distribution. If you set to "Best", the best performing ad is predominantly shown. Other ads in the campaign are occasionally also shown.

If you want to test different adverts (Banners or Native Ads) against each other, you have to create several adverts within the same adgroup.