[boost] [PBGL] Distributed BFS example : input and outpu

Hello, I am trying to understand how the example libs/graph_parallel/example/breadth_first_search.cpp works. I am not able to understand (neither find any documentation for it) the input and output of the program. To the best of my understanding a valid input to this program is graph/test/ weighted_graph.gr. What's the format for the input. And why does the output of BFS dependent upon the number of process in the mpirun argument. Would greatly appreciate any explanation or relevant pointers. -thanks Sandeep