I'm guessing - when you have some documents you often have it in many formats. In standard document management software you see document as single file - when you want to export your document as PDF it becomes other document in repository. In Keepmark your document can have many files - Invoice in DOC and PDF format? In Keepmark you can see them as single document.
Generally, behind this feature is a simple idea - Your resources are grouped into document using file names. The name before first dot in file name is a name of the document, the rest of document name are names of the resources in thtat document.