The WM/WMContentID attribute contains a GUID identifying the content.
Content is identified by Windows Media technologies by using three values: WM/WMCollectionGroupID, WM/WMCollectionID, and WM/WMContentID. These values identify the content, the collection to which it belongs, and the group to which the collection belongs. All three of these values are populated by Windows Media Player when metadata for the content is retrieved. You can have your application record these values and use them to identify content, but you should not change them if they are present.