The 'CopySelection' option is set on campaign level to determine which advert in a copy set is selected. If you want to test different adverts (banners or native ads) against each other, you have to create several adverts. You can then select whether the delivery should be even ('Even') or performance-based ('Best'). If you set to 'Best', ads with a better click-through rate CTR will be served at a higher frequency.