What that means is that the "Preview icon" file (which in your case was a smaller version of the full piece) should have something like "NSFW" included in it, as keywords don't immediately show up and the preview icon is the first thing a person sees before they click on it, so the icon having a visible indicator of mature content will immediately let people know that the piece is NSFW. There's also the "Adult" checkbox on the submission/edit page, though I've never used that personally so I have no clue if it works. Hope this helps.
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What that means is that the "Preview icon" file (which in your case was a smaller version of the full piece) should have something like "NSFW" included in it, as keywords don't immediately show up and the preview icon is the first thing a person sees before they click on it, so the icon having a visible indicator of mature content will immediately let people know that the piece is NSFW. There's also the "Adult" checkbox on the submission/edit page, though I've never used that personally so I have no clue if it works. Hope this helps.