Airlines manage their passengers using a Passenger Service System (PSS), which contains two components:

(i) Computer Reservations System (CRS), which accepts bookings into fare buckets received from an airline, holds passenger details and issues tickets
(ii) Departure Control Systems (DCS), which allow passengers to be accepted onto a flight, allocated a seat, issued a boarding card and receive printed bag tags and receipts.

A flights is said to move to airport control at a point approximately 72 hours prior to departure when CRS sends a message to DCS telling DCS to access the flight's bookings and make it ready to accept passengers for departure. After that point CRS will still accept bookings and issue tickets, which are transmitted to DCS, but DCS handles seating assignments.

At every point before the flight moves to airport control seat requests are made through CRS, but they are only requests and not actual allocations aboard an actual aircraft. With DCS, seat assignments are actual allocations onboard an actual aircraft that has been assigned to the actual flight. Allocations are much more final than requests, but can of course be changed through DCS itself should there be a need to do so.