It is unfortunate that there is no description for the matrix. Usually the field components will have the following dimensions:
x,y,z,f or t, m
where m=1 is for Ex, 2 for Ey and 3 for Ez. This is not very common. In FDTD its self the most often used is:
in frequency domain it is x,y,z,f
in time domain it is x,y,z,t
In python it may allow the dimension index begins at 0. I am not sure for this.