Ssrs 2010 document map

I am trying to understand how document map labels are working in an existing ssrs 2010 report since I need to make changes to the report.
There are 3 document maps that are setup in the report I am looking at, but there are 6 documents that show up when the report is excecuting.
Thus I am not certain how all this is 'wired' together to make these links in the report work.

Thus can you tell me how document link maps are setup and/or point me to a url that will explain how document maps work in an ssrs 2010 report?