looked here first: https://e621.net/help/show/ratings
still not clear

some examples from:
rating:s bikini
rating:s skimpy
and others

Edit: all used posts rated:s at time of posting

bikini top mostly cover's the center of the breast:
post #2022859 post #2021295 post #1969925 post #1998355

micro bikini/pasties/minimal breast coverage
post #1875049 post #508839 post #944702 post #1128382

skimpy costume
post #2011272 post #1992213 post #1773404 post #2006815

nipple outlines
post #1964891 post #1871171 post #1667891 post #1579688

rating:s bulge stuff
post #2010386 post #2021263 post #1305888 post #1172208

the stuff above but posed/with context(rating:q mistags?)
post #2014752 post #495367 post #1827730 post #1507177

...
post #1722910

revealing clothing doesn't = rating.

Nipple outline can be rated s or q depending on how obvious it is, and can be gray line. You can request mod decision for posts which it's hard to tell if the nipple outlines are obvious enough to be rated q.

post #1172208
This one is q not because of the bulge as the bulge isn't overly detailed but just shows that they character is male. bulges can be in any of the categories based of off detail and image scenario. This one was rated q due to the pubic hair and should always be rated q or higher

It's a bit of a mystery. There was a discussion a while back about what constitutes Safe with various clothing, poses, and details, but some of what the admins say is Safe is clearly defined as Questionable in the ratings guide, and some of what was said to be Questionable is what the ratings guide says should be Explicit. I don't remember that thread ending with any kind of explanation for the discrepancy, or what the "real" limits are supposed to be.

Versperus said:
Good examples and info

Watsit said:
There was a discussion a while back

thank you both, that was helpful

how do you request a mod decision on a post?

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also, are none of the last 5 images rating:q? because the rating wiki has these points under "Questionable":

• characters with revealing clothing with obvious erotic or teasing intent
• suggestive posing (such as butt shot) where focus is drawn to breasts, butt, or covered/obscured genitals

Edit: probably what you meant by this huh Watsit?

Watsit said:
but some of what the admins say is Safe is clearly defined as Questionable in the ratings guide

:/ ... guess I'll ignore that part for now