It is known that the flux of the Poynting Vector through a certain surface represents the total electromagnetic power flowing through it.
Then, its real part is the active power, while its imaginary part is the reactive power. Now let's consider an EM wave in vacuum: what do active power and reactive power mean?
From a circuit point of view, I'd say that the active power is that is dissipated in resistances, while reactive power is that is stored in inductances and/or capacitances. But in vacuum there are not materials with those properties. So what is their physical meaning?