Sets the order of a flight in the adChain. PassbackSortOrder is a transient value that can be modified by internal processes in the UI. Although the value may change, the actual sort order of flights in the adChain will remain consistent. Setting two flights to the same PassbackSortOrder will cause the flights to be chosen randomly. Setting PassbackSortOrder to 0 will cause the flight to appear as "--" in the UI, but the flight will still be part of the adChain. This is not recommended.