All websites have traffic that doesn't fit into any Ad Network. Usually, it's bot traffic or any kind of low-quality traffic like no cookies, proxy, VPN, etc.
​The Fallback URL is used for such kind of traffic.

For the Fallback campaign, we recommend using your own creatives only. Use it as a feeder for your websites. Go to the My Campaigns tab and click on [ + Import Creative ] green-lighted button displayed on the Fallback campaign.
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You may add more than one creative:

⚠️If your fallback URL gets more than 10-15% of your total traffic, it means your traffic quality is low or the system works incorrectly. Usually, it means a lack of linked Ad Networks.
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The minimum amount is 3, recommended is 5+. Add more Ad Networks and check the targeting settings of your campaigns.