A frequency control spec specifies settings for frequency capping. For example, {"event": "IMPRESSIONS", "interval_days":3, "max_frequency":1} means that in every 3 days, not more than 1 impression per user.
event
Event name, only IMPRESSIONS currently.
IMPRESSIONS
interval_days
Interval period in days, between 1 and 90 (inclusive)
max_frequency
The maximum frequency, between 1 and 90 (inclusive)