In order for Campaigns to be included in the forecast they must be Active, Approved or Reserved.
This means that if your campaign has Waiting for Approval or Killed as a status it's not included in the forecast.
You create a campaign but you can't finish the booking of the campaign because you have not yet received the creatives from your customer, you will need to change the status of the campaign to Reserved in order for it to be included in the forecast calculations.
If you fail to do this it can in some cases (depending on how many campaigns this happens to) blur the overview of available inventory thus leading to over-selling of your inventory. It is therefore very important to always change the status of a campaign if you want the campaign to be included in the forecast.