Originally Posted by us2

A "heavy" aircraft is one capable of a maximum takeoff weight in excess of 255,000 pounds, regardless of its operating weight at the time. Examples would include the 747, 777, 767, 757-300, DC-10, L1011 and DC-8-71.

UA often uses different flight numbers with ATC on flights that continue or originate domestically but have an international segment. This is presumably due to the fact that in the event of a delay, you could have two fiights in the air at the same time with the same flight number, s situation that the ATC computers cannot handle. There was a thread about this somewhere, perhaps a search will turn it up.