It appears to me that there is no documentation for the RngRun feature.

In the documentation of RandomVariable
http://www.nsnam.org/doxygen-release/classns3_1_1_random_variable.html
the following text appears:

"By default, the underlying generator is seeded with the time of day, and then deterministically creates a sequence of seeds for each subsequent generator that is created. The rest of the documentation outlines how to change this behavior."

however the rest of the documentation does not explain it.