A cross-reference is a relationship between a Document Type and one or more other Document Type(s). This allows you to view related documents by double-clicking on a word or a document region (an account number, for example).
Cross-referencing aids research by making all related documents readily available. For example, a vendor invoice could be configured to allow you to double-click the invoice to retrieve the corresponding purchase order. The purchase order could then in turn be cross-referenced with a corresponding requisition form.