Missing and empty data values

When you submit data for a control, some of the control data might not be available. For a control with missing data, you can do one of the following:

Exclude the ItemData element for a control for which data was not collected.
Set the IsNull attribute for the control to Yes if data was collected but then cleared, or to clear data for the control.
Note: If you specify the IsNull attribute, you cannot specify the Value attribute for the control.