To give another deviant a direct link to this FAQ answer, use the following code in your comment or forum post:
What is Mature Content?
Be aware that deviantART absolutely prohibits content which is deemed to be pornographic in content; the Mature Content tagging system DOES NOT exist to allow you to submit such content.
All deviations containing nudity, excessive violence, blood, gore, or other potentially upsetting or offensive material should be marked as Mature Content. Controversial pieces revolving around religion, political opinions and other similar sensitive subject matters should also be considered candidates to be tagged as Mature Content.
The deviantART administration reserves the right to assign the Mature Content tag to any submission which lacks it but is judged to require it.
More detailed Mature Content guidelines regarding nudity or erotica are here: CLICK FOR MORE