Sometimes a field's value is stored as null in the data source. You can specify a custom caption to display for fields that have a null value by setting the BoundField.NullDisplayText property. If this property is not set, null field values are displayed as empty strings (""). When a record is being updated or inserted in a data-bound control, if the user enters the value specified by this property (other than an empty string) for a field in a data-bound control, that value is automatically converted to null in the data source.