Forum moderators give out the R and other ratings. I imagine that the WM does this as well, when he is around, or thinks a post warrants it.
If you have a look in the different forums, you will see that some moderators are very (and IMO overly) generous with the rankings, and some are more moderate in their approach.
I use a few criteria for deciding which posts get an R or other ranking. The importance of the post to the people reading the forum, (posters can nominate posts for R's), The relevance of the post to the forum topic. The referencing used, and credit given where needed, and the quality of the content of the post, and comments, and replies.
Other moderators will use different criteria, I'm sure, but I can only comment on how I manage the rankings.