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.