2.4.4 Cells
The following sections specify the pre-defined cells that can exist for a sheet. Each cell is specified by a Cell_Type child element of a Section_Type, Row_Type, ShapeSheet_Type, PageSheet_Type, StyleSheet_Type, or DocumentSheet_Type element. Other cells can be defined through sheet extensibility.
The N attribute of a Cell_Type element MUST be equal to one of the values defined in the following sections, unless it is defined through sheet extensibility. The meaning of the attribute is specified in the corresponding section.