In order for an advertiser or campaign to appear in the list for inventory to a zone, the advertiser/campaign must have at least one banner which matches the zone size and type.
A zone's size is set on the main zone properties page. Any banner type can be linked to most zone types, unless the zone is set to the text ad type or the email/newsletter zone type. An email zone will only accept campaigns with an end date, and displays only one active image banner. A text ad zone is intended for text-only banners.
A local banner will automatically determine the banner size. An external, html, or text banner must be assigned a size.
If wanted, you can set a zone to accept any width or height of banner. For more information please see:
http://www.openx.org/node/263
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