When a cell doesn't contain any data and is between other cells that do contain data, it is called a missing value. When a value is missing from a column of numeric or date/time data, Minitab denotes it with an asterisk. When a value is missing from a column of text data, Minitab denotes it with the word "Missing".
If your data uses "-99" or some other value to identify missing values, you can use to change those values to asterisks. If you do not change the values, Minitab will use the incorrect values, which may result in misleading results.