In reality the 18+ tag doesn't need to be used in public roleplays since most of the content will be readable by people of all ages. GMs may choose to use it to give a guideline on the content that it may contain, eg: horror, violence, drugs, etc.

The tag is more for the 1x1 section and those who wish to engage in smut writing within the PM system. Since we don't allow sexual content on the public section of the board we limit it to where it is more private and it does not interfere with Google's AdSense. So it's really just a way to say you may be interested in an adult 1x1 without going into the details.