As of a year or two ago, United's scheme was approximately this:
UA1-UA2 Round-the-world flights (gone now?)
UA3-UA799 General domestic and Canada flights
UA800-899 Pacific flights (incl. US continuation segments)
UA900-999 Atlantic flights (incl. US continuation segments)
UA1000-1999 General domestic and Canada flights
UA2000-2999 Shuttle by United (gone now)
UA3000-3999 Code shares on other carriers
UA4000-6999 United Express (operated by contractors)
UA7000-7999 Unused? Charters? Cargo?
UA8000-8999 Reserved for ATC renumbering*
UA9000-9999 Special flights - charters, extra "passenger
protection" sections,
delivery and repositioning flights, etc.
Normally even numbers are eastbound, odd are westbound.
* When flights with a similar or the same number would overlap in an ATC
sector, one of the flights is renumbered, usually flight number +
8000. For example, let's say UA921 operates LHR-ORD-LAX (with a change
of equipment in ORD). If the inbound from London is seriously delayed,
UA may dispatch the domestic continuation (most of whose pax have no
idea "their" flight is delayed inbound, since there's an empty 767
sitting there at the gate), and the inbound LHR-LAX pax will be
accommodated on other flights. The domestic leg will be redesignated
UA8921 for ATC purposes so it does not get confused with the "real"
UA921 coming in from LHR.
These schemes have been upset by the termination of Shuttle by United;
it would be nice to know UA's current scheme, although I'm sure it is
somewhat similar.
--
Michael C. Berch
mcb@postmodern.com