The letter z has many definitions in X-ray astronomy. The intended one must be deduced from the context in which it appears:
The coordinate along the optical axis of Chandra. A standard NASA convention used in many missions.
The height above the plane of a galaxy, accretion disk or other flattened distribution.
The fractional abundance of "metals" (i.e. elements heavier than helium).
The redshift. When due to expansion of the Universe, generally used to give distance.