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availability is exclusive OR
availability is connected to an external system (external Adserver)
For more information on Availability scenarios :please check the link.
Therefore, after checking availability, you know if the preferred items are available and, if not, the maximum amount of available units.
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You can see the results for each item and you will also receive an email with the results (your administrator can turn the email notification off in the 7.2.1.1 Administration Settings).
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Clicking on one of your items, will expand it to show more details:
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