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
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.