Within a line item, you can set specific dates for each creative to run. That means that you could have periods during the run of a line item when no creative is running, or when there's no running creative of a particular size.
During these time gaps, the line item won't deliver. Instead, DFP will simply deliver creatives from other eligible line items.
Creative time gaps and delivery scheduling
Ad serving takes creative time gaps into account when pacing delivery for a line item.
For example, if a line item is booked for three days, with creatives scheduled to run on Day 1 and Day 3, but not Day 2, DFP will recognize that no impressions will be delivered on Day 2, and will divide up the booked impressions accordingly on Day 1 and Day 3.
Creative time gaps and forecasting
When you run a forecast, DFP takes account of time gaps in line items. (If you're running a forecast for a particular line item, only gaps in other line items are considered. DFP assumes that the current line item won't have creative time gaps.)
For example, a trafficker creates a line item, books 1 million impressions, and sets it to run for three months, from January 1 to March 31. She then adds two creatives. Creative A is scheduled to run during January, and Creative B is scheduled to run during March. There is no scheduled creative during February.
When a forecast is run for a different line item, the forecast estimates that approximately 500,000 impressions will be delivered in January and 500,000 in March, but none in February. The availability of inventory will be estimated accordingly.