This is a descriptor for the physical axis or set of axes
              corresponding to an array index.
              This data item is related to the axes of the detector
              itself given in DIFFRN_DETECTOR_AXIS, but usually differs
              in that the axes in this category are the axes of the
              coordinate system of reported data points, while the axes in
              DIFFRN_DETECTOR_AXIS  are the physical axes
              of the detector describing the 'poise' of the detector as an
              overall physical object.
              If there is only one axis in the set, the identifier of
              that axis should be used as the identifier of the set.