A commercial customer may order overglaze decorated ware due to the limitations of the manufacturer. E.g, there are number of companies that decorate glaze fired ware only. These are particularly popular with very small or exclusive foodservice customers. Others require very bright or metallic colors, which must be fired overglaze.
Please read RC Vol 1, page 350-351, especially the right column on page 351, for clarification on these processes.
Barbara