You can specify mandatory constraints for specific columns in workflow. This enforces population of the column with a not null value in order for the record to be approved. Mandatory Columns can be enforced for an object at each level of the workflow. Once enforced, the first approver at each level must enter a value in the mandatory column before the record can be approved.

Mandatory column for workflow is available for both data objects and conceptual objects. The enforcement of mandatory column for conceptual object is still at a data object/workflow rule level.