81dbfe30_essage_Sequence_Chart_Renderer__Meaning
[Source Attribute] URL [Meaning] 
When generating the image, colour the label in blue. If generating an image map, create an entry for the label to the specified URL. If the message sequence chart is to be embedded in Doxygen documentation, the URL maybe specified as \ref xxx where xxx is the name of some documented element; in this case a link to the element will automatically be made.
Mscgen: A Message Sequence Chart Renderer
Meaning