A checked bag disappears through a rubber curtain and reappears on a carousel, sometimes a continent away. The machinery in between is a sorting problem solved almost entirely by reading labels.
The tag is the whole system
At check-in the bag receives a printed tag carrying a unique number, the flight, and the destination. Everything that happens afterwards is a response to that number.
Scanners along the belt attempt to read the barcode as the bag passes. A tag that is folded, torn or facing inward may not be read at any of them.
Unread bags divert to a manual encoding station where a person reads the label and enters it by hand. That detour costs time, which is the resource the system has least of.
Sorting happens through mechanical decisions
Once identified, a bag is tracked by position rather than by continuous scanning. The system knows which tray or belt segment it occupies and when that segment reaches each junction.
Diverters push the bag onto the correct branch at precisely the right instant. A tray tilts, a paddle swings, and the bag leaves the main line toward its flight.
Because the decision is timed rather than sensed, a bag that slips, tumbles or sticks can miss its diverter and continue to the overflow line at the end.
Security screening sits inside the flow
Hold baggage passes through imaging equipment as part of the same journey. Most bags clear automatically and never leave the main path.
Bags that cannot be cleared automatically are routed to progressively more detailed inspection, and a small number are held for a physical search with the passenger located if necessary.
Each escalation adds minutes. A bag checked close to departure has little margin to absorb them, which is why late check-in raises the odds of the bag travelling later than its owner.
Connections are where bags are lost
A transfer bag has to be unloaded, read again, re-sorted and reloaded within the connection time. Short connections leave almost no slack for any of those steps.
When the inbound flight is late, the passenger may still make the connection on foot while the bag, which cannot run, does not. The two are travelling at different speeds.
The bag is then rebooked onto a later service and delivered afterwards. Most so-called lost luggage is simply delayed by one flight in exactly this way.
Reunion depends on the same number
When a bag and its owner separate, tracing relies on the tag number recorded at check-in and on descriptions of the bag itself.
Airlines share a common tracing framework so a bag found at one airport can be matched against a report filed at another, which is why the tag receipt matters.
Bags that lose their tag entirely become the hard cases, since they must be matched by appearance and contents rather than by the identifier the whole system depends upon.