This validator confirms that a specified field (which can be a custom field) has been set.
To add the Field Has Been Set Validator
Go to the transition of your workflow, switch to the Validators tab and then click on the Add Validator button.
Select JEP - Field has been set validator and then click the Add button.
Select the field which you want to allow the transition for.
That's it, the validator will now be added to the transition.
If the field has not been set but the user tried to perform the transition, the below error message will be displayed.
Do not forget to publish the workflow.