I know - I read your post. I meant that it's sometimes difficult to draw the line, and when posting on Tapastic, it might be better to err on the side of caution. At any rate, if there IS nudity, sexual activity or graphic violence in your comic, you need to put an NSFW-tag on it.
As for what constitutes "graphic violence", my rule of thumb is if you could get away with it in a regular action movie, it's probably okay to not tag it NSFW. But if you start drawing very detailed images of dismemberment, beheadings, intestines, etc., - you know, gore - you need to tag it as NSFW, and if your comic is full of that sort of thing, you need to consider whether Tapastic is the right place for you.
Tapastic's an American company, and so their policy most likely complies with the G to NC-17 ratings commonly used in the American film industry. Here's a Wiki-article on MPAA's rating system, with more in-depth info. If your comic would fall into the NC-17 category (i.e: clearly intended only for adults), you might need to consider hosting it on your own website, rather than on Tapastic. Even if you do put the content behind an NSFW-warning, the most extreme forms of depictions of sex/nudity/violence aren't permitted by Tapastic's Content Boundaries.
