Dear writers on AO3: "No archive warnings apply" is for your fluff, soft character, chill smut or story fics. It's for when there's nothing big/triggering and none of the major archive warnings show up in your story.
"Creator chose not to use archive warnings" is THE EXACT OPPOSITE. It is a Massive "enter at your own risk" sign indicating one or more of the major archive warnings or other nasty/gnarly/triggering tags may apply but would be a major spoiler to tag (e.g. horror/thriller fics where one of the major characters is killed). PLEASE don't use this tag if you mean "no archive warnings apply," they're not the same thing.
Sincerely, someone who just saw four T-rated fluff fics in a row tagged "creator chose not to use archive warnings" and had a minor heart attack.
Okay, I get what you're trying to do here, but this is not actually true.
"No Archive Warnings Apply" is used when none of the required archive warnings are present in the fic, correct. This is for fics which do not contain Graphic Depictions of Violence, Major Character Death, Rape/Non-Con, or Underage content.
HOWEVER. This emphatically does not mean the NAWA tag is only for "fluff, soft character, chill smut or story fics." A NAWA fic may contain balls to the wall raunchy smut, Blorbo's terrible horrible no good very bad day, massive violence not graphically depicted, relationships that are deeply unsettling to every outside observer within and without the story, whump, heartbreak, tragedy, and much much more. A fic being marked "No Archive Warnings Apply" is not a guarantee that anyone involved is going to have a good time — only that there is no Graphic Depictions of Violence, Major Character Death, Rape/Non-Con, or Underage. There is a reason AO3 also has ratings and other tags available to authors.
"Creator Chose Not To Use Archive Warnings" is, as you say, the "Enter at Your Own Risk" flag, but NOT because it means one of the Archive Warnings or other heavy content is necessarily present.
CNTW means precisely what it says — the creator elected not to use the Archive Warnings. It may contain all of them. It may contain none of them. It may contain weird edge cases where the author was unsure if an Archive Warning would apply. It might be a fic with dark, heavy, or disturbing themes despite the absence of those big four. It might be a perfectly pleasant T-rated fic posted by someone who simply does not philosophically vibe with mandatory warnings. (Those writers exist!) It may even be nothing but fluff imported through the open doors program from an older archive that didn't use any kind of warning system. CNTW offers no guarantees as to content; it simply stands as a signal to the reader that this work's metadata cannot tell them what lies ahead.
Now will many authors, out of preference, courtesy, or habit, choose NAWA for "softer" fics and CNTW for "darker" fics? Certainly. But this is neither universal nor required by the Archive. The warning tags mean what they say, no more, no less.
Yeah, I am CNTW for everything I write, as I am indeed philosophically opposed to mandatory warnings (I think CNTW was originally aimed at writers like me, but that was a long time ago, and my recollections might be hazy). Most of my output since 2016 has been romcom fluffy. Still CNTW! A few things that have come in through Open Doors might still have No Archive Warnings Apply accidentally set from the migration, but I fix those as I see them.
I tag as well as I can, for the most part, but I make no guarantees. Thus, CNTW.
(Would I ever use archive warnings? Only if I wrote something that hit Warnings Bingo.)
"Creator chose not to use archive warnings" means "NO SPOILERS"
It does not imply anything about the content of the story. All it tells you is that the author doesn't want to give the reader any hints about that facet of the storytelling, for whatever reason.
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Personally, I'm the kind of person who likes to know what I'm about to read before I read it so I don't waste time reading stories I don't like.
This applies especially to fanfiction.
If I'm reading original fiction, it's because I'm more open to experiencing something new and unexpected.
If I'm reading fanfiction, it's because I want a specific type of storytelling experience that the canon failed to give me, so I am looking for that specific thing in fanfiction.
If I am in the mood to read something with a happy ending, and the author doesn't disclose whether or not their story has a happy ending, I will skip to the end to check, because if I know that the ending is not the ending I want, it will ruin the entire story for me, and I will regret the time I wasted reading it.
If I am in the mood to read something fucked up and twisted, I want to be able to find what I'm looking for. I don't want to go hunting through a bunch mystery fic that might be innocuous fluff. That is also a waste of my time.
But that's just me.
Some people love fanfiction that surprises them. They have a surprise kink. More power to them. YKINMKATOK.