This is the only interface on the data sheet document class definitions that has specific functionality that allows the document to be related to a tag. It also stores the name of the relationship to the tag in the property SPFRelToDatasheetObj. This allows the snapshot functionality to navigate from the document to the tag to create view and XML files of the data sheet, which are then stored in the vault and related to the latest version for this revision.
SPFDatasheetDocType is used to match the SPFDatasheetType property on data sheet definitions to determine which data sheet definition is relevant to the document when creating a snapshot. An example value is IPD. These values are populated from the create method for the data sheet document.