Creative time gaps
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 doesn't deliver. Instead, DFP will simply deliver creatives from other eligible line items.
There can also be time gaps if you haven't yet added creatives to a line item, or if the line item is missing creatives of a particular size.