Data items in the ARRAY_STRUCTURE_LIST_AXIS  category describe
    the physical settings of sets of axes for the centres of pixels that
    correspond to data points described in the
    ARRAY_STRUCTURE_LIST  category.

    In the simplest cases, the physical increments of a single axis correspond
    to the increments of a single array index.  More complex organizations,
    e.g. spiral scans, may require coupled motions along multiple axes.

    Note that a spiral scan uses two coupled axes: one for the angular
    direction and one for the radial direction.  This differs from a
    cylindrical scan for which the two axes are not coupled into one
    set.

    Axes may be specified either for an entire array or for just a section
    of an array.