Hey, Wyckoff sites represent the multiplicity of a special position or general position. For example, a 8a site will generate 8 atomic position. Atoms will count for one half when situated on a face, one quarter on an edge, one eighth on a corner or as one in the unit cell.ou can do this for all structures. You can do this for all structures. 

These Wyckoff sites will depend on the space-group you consider, The more symmetry elements are considered, the more a general position will generate atomic position in the unit cell.  

Hope I helped you, if you need clarification. Just ask. 

So what does the 16c and 16d means'

maybe 16c means polyhedron which is combined with octahedral and tetrahedral

Robin Lefèvre Sir, could you provide any reference/books/text for this.