The 280mm Rocket Artillery is a doctrinal ability that barrages a specified area with very powerful (off–map) artillery with a good area of effect. There is a short delay between designating a target and it being barraged, so it is advisable to target base buildings or defensive structures, as units could easily have moved on before the artillery arrives. Another restriction is that the targeted area cannot be out of sight range of any unit or building, thus it is very hard to call the artillery down on enemy bases.
Good targets include weapon teams, AT guns, on-map Howitzers or Priests, and large concentrations of troops, especially when you can time the rocket barrage so it hits right after your enemy retreats most of his forces, as in that case, they will be unable to escape the rockets landing at their Headquarters. Bear in mind that it is not terribly effective against many British emplacements, so save your munitions in that case.