International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. C, ch. 9.7, p. 904

As developed so far (Belsky, Zorkaya & Zorky, 1995), structural classes relate primarily to homomolecular structures – structures in which all molecules are the same. Nevertheless, they are important in a study of the spacegroup distribution of molecular organic structures, as structures belonging to the same spacegroup type but to different structural classes are found to have very different frequencies. The germ of the idea is implicit in Kitajgorodskij's subdivision of his four categories by molecular symmetry.
In its general form, the symbol of a structural class has the form where is the standard spacegroup symbol, n is the number of molecules in the unit cell, are the symbols of the pointgroup symmetries of the Wyckoff positions occupied, and are the numbers of occupied Wyckoff positions of those symmetries. An example will make this clearer. There is a structural class This indicates that the space group is and that there are 32 molecules in the unit cell occupying four positions of symmetry mm, four of symmetry m, and one general position. On consulting the multiplicity of the special positions for this space group in Volume A of International Tables for Crystallography (Hahn, 2005), one finds that the 32 molecule total is accounted for as (4 × 2) + (4 × 4) + (1 × 8). If (as is usually the case) the square brackets are unnecessary, they are omitted, as in Occasionally, two distinguishable structural classes will lead to the same symbol. As yet, Belsky, Zorkaya & Zorky prefer to deal with this problem on an ad hoc basis, rather than by attempting to devise any general rules.
Belsky, Zorkaya & Zorky divide the structural classes into six groups, in accordance with the number of examples found. The groups are `anomalous' (up to five examples, 199 structural classes); `rare' (up to 19 examples, 55 classes); `small' (up to 49 examples, 24 classes); `big' (13 classes); `giant' (eight classes); and `supergiant' (six classes). The last three are not explicitly defined, but examination of the tables shows that the dividing line between `big' and `giant' is about 250, and between `giant' and `supergiant' is about 750. All these statistics, of course, are subject to modification as the number of known structures increases.
References
Belsky, V. K., Zorkaya, O. N. & Zorky, P. M. (1995). Structural classes and space groups of organic homomolecular crystals: new statistical data. Acta Cryst. A51, 473–481.Hahn, Th. (2005). Editor. International tables for crystallography, Vol. A, Spacegroup symmetry, fifth edition. Heidelberg: Springer.