(1) purchase ticket for a certain number of days before departures (typically 14 days, 21 days and possibly 28 days) in advance or within a fixed time from the date of booking.
(2) a minimum stay requirement (seven days or over a Saturday night).
(3) typically non-refundable and may have many other restrictions – flight, departure date, seat on plane
(4) no stopovers.
Cancellation of an APEX fare trip usually results in higher than normal penalties.
In effect, all airline fares (and airport taxes) are subject to change up to the time a ticket is issued (but not afterwards).