Undefined Cells in the Sheet
To reduce file size, cells that don't contain values or formulas and aren't referenced by formulas in any other cell are considered to be undefined cells. Such undefined cells don't appear in the BIFF file.
For example, if a worksheet has a value in cell A3, and the formula =A3+A4 in cell B10, then the only defined cells on the worksheet are A3, A4, and B10. No other cells need to exist.
Using this technique, entire rows can be undefined if they have no defined cells in them. In the preceding example, only rows 3, 4, and 10 are defined, so the file contains only three ROW records.