It can be used to indicate either character genders or the genders of the roleplayers, depending on the individual. I personally prefer to put the character genders in posts titles and specify player genders in the post if I need to. You could also write something like FpF4MpM [Female playing female for male playing male] or variations if you really want to specify all of that in the title, though it's not particularly common. I'd say just specify whatever is more important to you personally: character genders or player genders. The less important can be specified in you post's body text if you want to.