For Fe-I we see that there are tree 4-fold rotation axis (x, y and z) and thus we get that a rotation axis is the z-axis of PAS and that eta=0
For Fe-IIa we see a 4-fold rotation axis and two 2-fold rotation axis. So a 2-fold rotation axis can be chosen as z-axis of PAS.
For Fe-IIb we have a 4-fold (or more) axis and two 2-fold rotation axis. So again a 2-fold rotation axis can be chosen as z-axis of PAS.