Something new DrJ has come up with and still a work-in-progress. He has stated that he hopes to tie some type of rewards system to the rank (maybe xtra discount in the BPL OnLine Store???). He wants to give something back to those who help make BPL.com successful.
The general consensus over the last month or so has been that most people are choosing to largely ignore theirs and others ranks. Though some people find them useful.
It is a "top secret" algorithm, so we don't know precisely how it works, but one key to its function seems to be length of time as a BPL Member. This would be consistent with one of the goals of the ranking system.
There are two very long Threads, IIRC, that have many Posts about this new system.
Lastly, it is logarithmic in nature (like decibals and the Richter scale for measuring earthquake intensity), so a score of 3.0 is really ten times "higher" than a score of 2.0.
Hope this info helps to answer some of your questions.
Did you SEARCH the FORUMS using the small text Blue Link entitled "Search Forums"? I just did and after three "hits" in this Thread comes the info that you are looking for. Try the SEARCH PHRASE "BPL Rank" (w/o the dbl-quotes).