The interaction that receives the credit for a conversion. It's generally the most recent click, or if there's no click, the most recent impression.
If you override the lookback window in a report or change the maximum gap between interactions, this may change which interaction receives credit for the conversion. The new value will appear as a separate “Recalculated attribution interaction” field, so make sure to include that dimension in your report as well if you want to see the results of your changes.
