Certain airlines, on certain routes (mainly long haul) issue an allocation of discounted tickets that they make available through selected agents. This is to enable the airline to maximise yield as to sell all seats at the full fare is highly unlikely.
These net tickets do have certain restrictions attached to them. Certainly they can only be used by the issuing airline, and they will have a penalty of £50 or so to change. They can not be refunded.
However when one considers that in many cases 50% can be saved off the schedule fare (£2,500 to New York in Business Class instead of £4,200), these tickets are well worth considering.
Millennium Corporate Travel has net ticketing agreements with approximately 20 airlines.