Targetings and limits
If the campaign that you're making a forecast for has limits or targetings that reduce the amount of available inventory and explicitly affect the number of impressions, they'll be listed under the forecast.
The list contains the potential number of impressions that the campaign loses due to these limits or targetings:
Note
If there is no list of targetings and limits, it means that the forecast campaign doesn't have any settings that explicitly affect the way it runs.
If the forecast shows that the campaign may not be completed, you can simulate how disabling targetings or limits will affect the campaign. To do this, exclude a limit or targeting from the forecast by clearing the 
checkbox in the appropriate line and update the forecast. The line with the excluded parameter will turn gray (meaning that the parameter is not included in the forecast).
You can't disable targetings and limits when creating a campaign, but you can modify them in the settings.
For recommendations on what to do if your campaign might not be completed, see Forecast analysis.