as far as i know... Excessive Violence + Blood automatically make your game rated R.
also if the Strong Language is frequent it could also make your game rated R.
Restricted
R - Restricted - Under 17 requires accompanying adult over the age of 18. The person is required to stay with the child under 17 through the entire movie, even if the parent gives the child/teenager permission to see the film alone. These films may contain mild or implied sex scenes, prolonged nudity, strong violence often with blood and gore, strong horror scenes and explicit/illegal/prolonged drug use. A movie rated R for profanity often has more severe or frequent swearing than the PG-13 rating would permit. An R-rated movie may have more blood, gore, drug use, nudity, or graphic sexuality than a PG-13 movie would permit. Some R-rated films have an "unrated" DVD release with scenes of violence, sexual material, or profanity that have been edited from the original cut. At its inception, the R certificate permitted patrons aged 16 and older to attend unaccompanied, but this was raised to 17 in the 1970s.
hope this help.
edit: Tiamat-86 is correct... it's mainly for movies... game use ESRB rating:
E, E10+, T, M, A0, Rp.
i don't know about forum rating though...