Here are the "rules" as I understand them:
UA 1-1699 - mainline UA
* UA 1-99 Transcon/Premium Transcon
* UA 800-899 Transpacific and Latin America except MEX
* UA 900-999 Transatlantic
* UA 1000-1099 Mexico City and Canada
* UA 1400-1599 Ted
* UA 1600-1699 Caribbean and Int'l Leisure (CUN/GCM/AUA/BDA) including STT/SJU
UA 1700-4999 (approx). Various codeshares including US Airways, Mexicana, Asiana
UA 5200-5999 UAX op. by Air Wisconsin
UA 6000-6999 UAX op. by Skywest
UA 7000-7999 UAX op. by Atlantic Coast and Mesa
UA 8000-8099 UAX op. by Trans States.
Beyond - some other codeshares including Lufthansa, Air China, Air Canada...
And the rules above for westbound/eastbound...
------------------
Thank you for flying and staying UNITED. Your business and support is highly appreciated. Enjoy your flight and spread the word!