On the Properties panel for input objects, there is a check box indicating whether or not the object is considered mandatory. Using this property is only recommended if you have a short, single-page app without complex behaviors.

Check this property to indicate that a field is required upon submission of the app.
Click the ellipses  button to the right of the Mandatory answer box to configure the requirement across roles and stages.
 button to the right of the Mandatory answer box to configure the requirement across roles and stages. 
|  | Mandatory checking can also be achieved using behavior. See Check Mandatory behavior command for more details. | 
See also: Selecting a State
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