When a document is submitted to a workflow, it is normal for the revision to pass to other workflows. To enable this, the ISPFWorkflowItem interface is realized by the document revision classes. A typical workflow includes the review, approval, and sign off of a document revision. If it is necessary for a document master to be passed through a workflow, for example to manage the creation of new revisions, then this interface could be realized by the document master class.
See Workflow Model.