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From my perspective.

Safe = like admin2 said, anything that can view publicly at work, the library or at work.

Questionable = See through, not showing any vaginal area that is purposebly exposed, partial nipple showing such as post #54294

Explcit = Tentacle, sex, bukkake, and what other obscene exposure.

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Something such as Post #54254 is obviously not safe for work, but as long as viewing it privately or at home it's just questionable.