LOS question

By San, in Tide of Iron

The Rulebook states, that when two units are of different elevations, the attacking unit mey be able to fire ''over" some blocking terrain between the units.

Does it mean, that the lower unit also may attack the higher one (if the lower unit is not located in a blind hex), or only higher units can see over blocking terrains?

Thanks a lot.

LOS is always both ways.