I am trying to follow this tutorial from the FLEUR DFT code's official website. The tutorial goes over calculating the ground state energies for the ferromagnetic and antiferromagnetic configurations of fcc Fe. I understand that one needs to have at least two atoms in a primitive cell to have an antiferromagnetic order (for example, for the case of bcc Fe, one converts one atom-bcc to two atom simple cubit lattice structure).
However, here looks like they are converting a two-atom fcc unit cell into a tetragonal cell (Fleur manual says tp refers to simple-tetragonal). Unfortunately I'm not able to see how this can be accomplished. For convenience, I'm attaching the part of the code where they write this.
Any help in understanding how this can be achieved would be greatly appreciated.