In Sim4Life, the complex field S(x,y,z,f0) is (one half of) the cross product of the complex fields E(x,y,z,f0) and H(x,y,z,f0), both of which are the complex representation of harmonic fields (actually S = 0.5 E cross H*, where H* is the complex conjugate of H)

The time-averaged Poynting vector (if you want the power flow) is given by Re(S)

See the Wikipedia article (https://en.wikipedia.org/wiki/Poynting_vector) for more detail.

EDIT: this post was corrected to reflect the notations used in Sim4Life, namley that S(x,y,z,f0) is already the time-averaged quantity and includes the factor 1/2.