Under the pre-AC/CP system, which is still mostly intact (and I'm sure there may be an error here or there):
0000-0099: Asia Pacific and South America (inherited from CP)
0100-0199: Transcon
0200-0299: Western, but largely western triangle rapidair
0300-0399: Transborder
0400-0499: YYZ-YOW/YUL rapidair
0500-0599: (Can't remember, but I don't think this series is used)
0600-0699: Eastern
0700-0799: Transborder
0800-0899: Europe, Middle East, India
0900-0999: Caribbean
1000-1399: Domestic (largely transcon) and transborder overflow numbers (began appearing following AC/CP merger)
2000 series: Used as replacement flight numbers
5000 series: UA codeshares
6000-6099: AC Tango
7000 series: Regional
8000 series: Mostly regional
9000 series: Codeshares with other airlines