A
zone can have another zone as default or secondary. In case no
ad could be chosen for the primary zone, the system will try to fetch ads from the default/secondary zone.
Chain of Zones
You have the flexibility to create a chain of zones for a more advanced ad serving sequence. For example:
Zone B is secondary to
Zone A and
Zone C is secondary to
Zone B. The ad flow will be
A -> B -> C.
How do I show default/house ads after serving all premium inventory?
Because premium ad inventory brings in more revenue, you would like to finish serving these ads before showing house, remnant ad inventory. In order to do this, you would need to create two zones: Zone
RemnantHouseAds is the secondary/default zone for zone
PremiumAds.
Zone PremiumAds links to 3 premium ads (X,Y, and Z). If all ads in zone PremiumAds cannot be displayed because of their restrictions (i.e. ad expires, impression quotas reached, geotargeting failed, etc.), the system will automatically look for ads in the secondary zone RemnantHouseAds.
Considerations
All serving settings, including ad dimensions & all restrictions, are independently applied to ads in the secondary/default zone. Therefore, if no 468x60 ad in zone
PremiumAds can be served and you do not have any 468x60 ad in zone
RemnantHouseAds, an error message will be displayed.
Features 
