The code identifying the class to which this reflection has
been assigned. This code must match a value of
_diffrn_reflns_class.code. Reflections may be grouped into
classes for a variety of purposes. For example, for modulated
structures each reflection class may be defined by the
number m=sum|mi|, where the mi are the integer coefficients
that, in addition to h,k,l, index the corresponding diffraction
vector in the basis defined for the reciprocal lattice.