It is useful for users to be able to close a form before they have finished it in case they want to stop work on it and finish it later.
However, if your project uses multi-page forms, there is a chance that users will close the form after the first page, incorrectly thinking that they have completed it.
You can control whether a form should check that a user is sure they want to close a form when the form is not yet complete.
To do this, go into the Form Editor (See Editing Forms) and click the Edit button next to Settings on the Form toolbox tab.
Set Confirm Close to Yes
Click Close and the Close again to close the Form Editor.
Now when users try to close a form, before it is finished, they will get a confirmation screen like this one: