Specific orbits (732km with a 28 degree inclination and 130 degree longitude of the ascending node), but impacts also instantaneously spread debris out over anything close to any of those numbers (700-760, 27-29, 125-135); most debris get spread not in a normal distribution but in a bimodal distribution between the orbital elements of the two impactors, with an additional perimeter of debris kicked out with some random modest dV in every direction from the explosion according to how much impact energy they absorbed. To start with.

But then, debris start to decay, and they decay unevenly, polluting all orbits lower than that. As this debris spreads to a crowded lower orbit it generates additional collisions.

Orbits are cleared to lower orbits according to mass cross section at roughly 10x the rate for every 100km altitude drop. So everything that decays from an accumulation of higher orbits has to eventually pass through a 300km circular orbit, but it's relatively safe because it spends so little time there.

Going full exponential cascade at 1000km might increase the number of impactors by a millionfold, though, which then proceed to rain down over the years on lower orbits.