When the ticket is issued electronically, a copy of all the relevant ticket details are transmitted to the airline's reservations and/or DCS computer.
One of several major advantages to e-ticketing (other than not having to worry about losing your ticket) is that you can often bypass the huge lines at checkin, by using the automated kiosks. United calls theirs Easy checkin. You just put a credit card in the machine, so it can read your name, and type in your destination. The machine will find your booking, and then guide you through checkin, eventually spitting out a boarding pass. If you are traveling with bags, you can either check them curbside, or in many airports UA
has automated kiosks at a special section of the main counters, where staff will take and tag your bags.
Its a brilliant system. I just wish all airlines were up to speed and could interline equally. Some have been faster than others in adapting to the new technology.
It's been swell, but the swelling has gone down.