From the e621 site:

Explicit

Any image where the vagina or penis are exposed and easily visible. This includes depictions of sex, masturbation, or any sort of penetration.

Illustrations with no exposed penis, but containing a sheath (specially posing) have been rated either "explicit" or "questionable". Although it depends a lot on the viewer, it seems a bit odd having 2 types of rating for similar pictures.

Is there some kind of unspoken rule that might help decide on which rating to submit it under? Just wondering really.