The age rating is 15+. If you want to see everything then the rating should be 19+ usually it has a red bar on the cover. I really don’t understand the difference, I’ve been googling around trying to figure out how this content is considered suitable for minors. I don’t know sigh…
Basically publishers in KR are provided with a rating that the producer in charge of the work (manhwa, novel, web novel,) is to abide by whilst working to complete the story. It’s a set ruled based on how works cannot be edited or altered regarding the mature content. Those legacy media conglomerates SBS, KBS, Arirang etc don’t like publishing mature content on a wide scale so they use tactics such as these to corner and exclude mature content from the general public.
To you question why is it suitable for teens and the other is not is because each publisher can designate how much content they can push without it being 19+ rating. A loophole basically which is why you have manhwa like this and Despite the risk showing full blown sex scenes with no nips
TLDR; these corporations do this essentially because they don’t want their purity market to be overdone by mature work. And they’re very nit picky with what they deem mature and not mature. If that made sense
What is up with authors and not drawing nips it looks so weird without them