It's pretty much per the airline. There's no set standard. For instance, for UA 800's are trans-pacific flights. 900's are trans-atlantic. UA's flight 1/2 used to be the around-the-world flight. Nowadays it's operating as ORD-HNL-ORD. NZ's flight 1/2 is AKL-LAX-LHR and back.
Interesting story for any of you guys possibly watching the U.S. sitcom "Lost." For those that don't, it's about castaways on an island from a plane crash routing SYD-LAX on a fictitious airline called Oceanic. The flight they were on was numbered 816. Needless to say, UA's LAX-SYD-LAX flight for years and years was 815/816. About two years ago, when the show first aired (they are currently on their 2nd season), UA changed the number on that flight to 839/840. At the time I wondered why, as I have been flying this route since 1999 and has basically had the same flight number since they acquired the route from PA in the mid 80's. I figured the coincidence that ABC studios decided to use the same flight number (same route no less) had a lot to do with that. What are the odds that the creators of the show just randomly chose that number?