Goat, is your question AA-specific?

If so, here are some ideas -

AA tries to assign flights in 'ranges' and 'pairs' within those ranges. Some flight numbers have been 'retired', due to aircraft crashes. This then throws out the exact pairing scenario.

On American, Eastbound and Northbound flights are even-numbered. Westbound and Southbound flights are odd-numbered.

3-class transcons and Hawaii flights are generally in the 1-39 range.
Transatlantic/transpacific flights are generally in the 40-199 range. However, there are some domestic flights in this range too. 200-299 are mostly non-stops.
300-2999 are AA flights elsewhere in the system. 899-999 tend to be Caribbean/South American flights.

Eagle are 3000-5999. 6000-7999 are codeshares. 8000-9000 are reserved for AA internal use (test flights etc) and 9000-9999 are primarily used for ferry flights, off schedule ops, extra sections and so on.

Does that help a little?