The YAL-1 (aka, Airborne Laser) was designed to hit TBMs (smaller tactical ballistic missiles) during the boost phase, shortly after clearing their launch towers.
The laser is focused on the thin skin of the missile body, burning or weakening the structure just enough to cause a catastrophic rupture.
That's the measure of success for this program, killing a TBM from a standoff range of several hundred miles.
My agency played a big role in the development of the YAL-1, and our inside joke was that they may as well paint a big bullseye on the side of that converted 747-400F.
In the end it was $6 billion over budget when killed, nowhere near operational or even statistically successful.
There were even threats of Arms Control Treaty complaints enroute, because that big turret could be swiveled downwards to engage ground targets or upwards to engage satellites.
The Navy has picked up the ball now with their Free Electron Laser program.