This tag is defined as artworks that are above the R-17 rating, but only one level below R-18.
Also considered to be "barely safe", using this tag will mean browsing these artworks is not safe for viewers below 18 years of age, even if it's not entirely naked and doesn't clearly show any sensitive parts.
When tags like these are attached, people may question the credibility of the artwork. Some may even find it offensive so one should be careful whenever necessary.
Regardless, using R-18 is still recommended even if you think the submission is still on the safe zone. Because not everyone thinks the same as you do, so a proper classification is highly recommended.
While there are vague illustrations on what considered barely safe, there are also illustrations that actually show sex organs, and some illustrations doesn't show them too.
By the way, using R-17.9 on your own submission works the same way as using R-17.99 or 17.999. Still, it is better to label it R-18 instead of being vague.
Though it is still entirely your decision to decide what is consider safe, or not.