Originally Posted by robb
I suspect that what is a matter of perogative (and I think the perogative is UA's, not ATC's) is whether to call a flight UA 8XXX or UA 950A.
Checking FlightAware, it seems that most or all of the eastbound flights into IAD that continue as international flights are using the format UA 9XX/alpha, and the alphas seem to be unique (not re-used); the eastbound flights into ORD that continue as international flights all seem to be UA 8XXX.