 Solution

Reflection through $z=0$ interchanges the two Se height classes but, by itself, leaves their projected positions wrong. The required parallel translation isFor example, $(1/2,0,\eta )$ maps to $(1,1/2,-\eta )$, equivalent to the second Se site. The two Fe sites are also interchanged. This is a diagonal, or $n$, glide; adding an integer lattice vector gives the same fractional translation class. Its translation is expressed in the conventional primitive tetragonal lattice coordinates, not in a smaller Fe-only cell that would discard the selenium height pattern.