A quick glance around the forum suggests that the number of postings required to progress through the ranks of membership may be as follows:
50 = Junior, 100 = Full, 250 = Senior, 500 = Supreme.
It is all based on quantity (number of postings), rather than quality. These are set by the forum administration, which may also assign specific groupings should it wish. There are a number of members who may wish for rank, and even continued membership, to be decided on a more democratic basis.
To offer a personal view; as the number of posts are shown to give readers some impression of the scale of a member's contributions and the content speaks for itself, the fact that the former is turned into a label need not be a matter of a concern.
If looking to make a few postings to gain promotion, discussion of this item may be of interest to some members -
http://www.theyworkforyou.com/wrans/?id=2010-07-06b.5363.h.