The Fe-I site has one three-fold orientation axis pointing towards the N site. Due to this eta = 0 and this axis can be chosen as the z-axis of the PAS.
The Fe-IIa/b site has one two-fold orientation axis pointing towards the N site. This axis can then be chosen as the z-axis of the PAS.
Note: I was not sure if rotating around any other axis that was not pointing to the N site could be considered a proper symmetric axis as you’re rotating the whole cube around this axis. Could somebody tell me if this is the case or not.