How is an ItemId different from DocumentId in Sharepoint Server?
documentId is unique to the content repository, what is the need for
DocumentId is populated by "Document ID Service" feature in SharePoint that can be turned on and off at the site collection level which means that the field may or may not be populated. When active, it provides a way to locate the document regardless of its current name or location. Item Id provides a unique integer for the document within a specific list and is one of the more common ways to reference a document that does not rely on name of the document, which can change.