Currently if forum posts are numerically rated, the forum grade is the average score of all a student's rated posts. For example, if a forum is worth 10 pts and a student post is rated at 9 and then the same student posts again and it is rated at 3, the student grade is 6 – students can lose points for additional posting.
An optional Additive option would allow the score to be the sum of all the student's rated posts, limited to the max value set for the forum (recalc if forum scale is changed to different max value).
This would also allow for rating each post 1 for those who simply want a count, though Count could also be an addtional rating type and would be automatic if selected, without putting anything in (or altering) the rating field.
Yet another possible rating type would be Highest (to take an analogy from quiz scoring).
Ideally none of these would affect the individual ratings, just the score that is sent to the gradebook.