UserValues are a property of every instance of EnsLib.EDI.Document objects (e.g. HL7 Messages). They allow storing of e.g. Metadata alongside the message using dtl and retrieve the information stored in there for example in routing rules. Unfortunatly the contents of this array can only be displayed when opening the message object in terminal.
The idea is to display those UserValues on the messages property tab in Visual trace. It should be quite easy patching EnsPortal.Utils.cls to display those information.