...
A workflow action constraint puts a restriction on the transition from one state to another for content items in a workflow. The constraint is based on a specified property or condition belonging to a content item. For example, a constraint can prevent a content item from transitioning from the "Planned" to the "Published" state if it does not have a title or if it does not have an expiration date assigned to it. A constraint can be global and apply to all content types or you can select the specific content type to which it applies.
...