Possible delivery rates for a LineItem, which dictate the manner in which they are served.
|Line items are served as evenly as possible across the number of days specified in a line item's LineItem.duration.
|Line items are served more aggressively in the beginning of the flight date.
|The booked impressions for a line item may be delivered well before the LineItem.endDateTime. Other lower-priority or lower-value line items will be stopped from delivering until this line item meets the number of impressions or clicks it is booked for.