It indicates that you have a bus without a name and therefore no nets associated with the bus. You must give a bus a name and some nets that belong to the bus. It may be a name that defines the contents such as Data[7:0] indicating the bus has 8 nets, Data7 to Data0. Or you may give it a descriptive name and use Bus Contents (Setup - Settings - Bus Contents) to define the member nets. For example bus name = DataBus, bus contents DataBus=Data[7:0]. In which case DataBus has bus memebers Data7 to Data0. The advantage with bus contents is that you can contruct more complex lists of bus members, as an example DataBus=Data[7:0], DataEn, Clock, Reset. In which case all of these nets are available when you connect to/from the bus (Rip Nets etc). For more information look in the DxDesigner User's Guide.