A user specified cell format, which may be reused across many cells. The constructors takes parameters, such as font details and the numerical date formats, which specify to Excel how cells with this format should be displayed. Once a CellFormat has been added to a Cell which has been added to a sheet, then the CellFormat becomes immutable (to prevent unforeseen effects on other cells which share the same format). Attempts to call the various set... functions on a WritableCellFormat after this time will result in a runtime exception.