An airline's reservation system knows who intends to fly. It does not know who actually turned up, which seat they took, how many bags they handed over, or whether they walked through the gate. That gap between intention and fact is where a departure control system lives.
Local DCS manages the final phase: accepting the passenger, checking documents and special handling, assigning a seat from the cabin layout, recording baggage pieces and weight against the passenger record, controlling boarding at the gate, and closing the flight out with a manifest.
The word local matters. A station without a full host DCS — a smaller carrier, a handling agent covering several airlines, or any airline on a day when the host is unreachable — still has to depart the aircraft. A departure control system that runs at the airport rather than at the other end of a link keeps the operation moving when the link does not.
It also means the numbers stay together. Checked-in, boarded and no-show counts change continuously in the last hour, and every one of them has a consequence: a no-show is a bag to find, a seat to release, and a decision about whether to wait. When those counts and the baggage figures sit in the same record, the gate agent and the ramp are reading the same flight.
Close-out is the moment all of it is fixed. The manifest and the departure figures become the account of what happened — and, downstream, the basis of what gets billed.