A zone can have another zone as default/secondary. In case no ad could be chosen for the primary zone, the system will try to fetch ads from the default/secondary zone.
Example
Zone B is the secondary/default zone for zone A. Zone A links to 3 ads (X,Y, and Z). If all ads in zone A fail their restrictions (such as: serving period expires, impression quotas reached, geotargeting failed, etc.), the system will automatically look for ads in zone B. The ad size settings & restrictions are independently applied to ads in zone B. Thus, if no 468x60 ad in zone A can be served and you do not have any 468x60 ad in zone B, an error message will be displayed.
Chain of Zones
You have the flexibility to create a chain of zones for ad serving purposes. Zone B is secondary to Zone A and Zone C is secondary to Zone B. The flow will be
A -> B -> C.
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