Inventory forecasting estimates the maximum number of impressions a campaign can get based on all the traffic (including RTB) from the previous calendar day.It takes into account the format, campaign type and extended targeting of the campaign.While making the estimation it does not take into account the following criteria:Weekly distributionTotal campaign budgetZone SubID cappingOther "special" categories.One thing to keep in mind is that this tool displays an average number of inventory available for the offer and as each campaign is individual  the available number of inventory may vary as it depends a lot on the targeting set by the advertiser.