Newbie Question #4 CAUTIONs and Filters By Audience

This should HOPEfully be the finally one.

Anyways, first what is the difference between CAUTION: Sex / Sexual Scenes and CAUTION: Not Work-Safe ?

Second, I am curious which of the following would apply in a story I am working on. Mature Subjects or EXPLICIT CONTENT ? The reason I ask is because while the story will have a lot of sex scenes and they will be depicted explicitly. They will not be in every chapter or even the main and/or sole focus. Heck it might take me awhile just to build up to the one as such I am honestly not sure what to apply.