When this block is given, the polarisation of the TDExternalFields is understood to be defined in terms of Miller indices. This block define the corresponding basis, by defining the reduced coordinates of the X, Y, and Z high symmetry points, such that the code can do the corresponding transformation.
For example, in an FCC crystal with the conventional primitive cell, the following input allows to define the polarization in terms of Miller indices
0.0 | 0.5 | 0.5
0.5 | 0.0 | 0.5
0.5 | 0.5 | 0.0
Indeed, in this case, the reciprocal lattice vectors are (-1, 1, 1), (1, -1, 1), and (1, 1, -1) in units of 2*pi/a. This directly gives that the  direction correspond to the x direction,  gives the vector (1,1,1), etc.
hamiltonian/lasers.F90 : 313
if (parse_block(namespace, 'MillerIndicesBasis', blk2) == 0) then