Sorry - I misread the OP and answered a completely different question! However, for anyone who may be interested, I'll leave my post untouched, just in case someone thinks of the security angle!

The answer is that the check-in agent enters your "Onward Carriage" details into our Departure Control System. What we have to do, for example, is answer the following questions from a "pop-up" window:

Flight Number BA2946 (Must be IATA 2-letter airline code - if Charter flight, eg MON2247 DCS rejects on-carriage)
Desstination EDI (Must match flight no destination or DCS rejects on-carriage)
Date of Travel 15 JUN 06 (must be same day or within 24 hours of first flight)
Ticket Class M
Status OK (If not "OK" status DCS rejects on-carriage)

DCS, however, does rely on human input so, "Garbage In, Garbage Out" applies. British Airways has always relied on check-in staff to have a quick, accurate eye for detail but as the demands on us grow (have you seen the queues in the terminals lately with only a skeleton number of staff working), we do find errors being made!

The worst one, and one that the Trades Unions have been at loggerheads with Marketing since its inception is the "Fast" Bag Drop. The use of the word "Fast" implies something it can never deliver - unless the agent manning the desk cuts a few corners and BA accept the odd error! .........But, that's another story!