Explain to me the Tag System of Ao3

People use tags in two ways: To search for things they want to read, and to block out things they DON'T want to read. I recommend using AO3's tags to try and find fic you want to read, and try excluding things you don't like, to see how the system works. Use that to make your decisions.

At the end of the day, no tags are mandatory besides AO3's major archive warnings, which are spelled out in their FAQ, so you can choose if you want to be a minimal tagger or put as much info out there for readers as possible.

Something you may want to avoid is using your tags like authors' notes - funny commentary and such clogs up the page and makes reading the rest of the tags more difficult, and it puts some readers off because of that.

Here is the guide on tags

I would say the main takeaway when it comes to tagging is to be thoughtful when it comes to the rating and the warnings. The guide has some pretty clear outlines on each category.

Outside of the major warnings, if you think you have something unexpectedly triggering in your fic, you may want to warn for it in the tags to avoid getting comments from upset readers. (examples include things like eating disorders, self-harm, racism, miscarriages, ect.)

If you have a romantic/sexual pairing/relationship you'll want to use the character/character tag. Platonic relationships are tagged character & character.

Otherwise, you can use any of the canonical tags (the ones that auto-fill) so people are able to find your work based on the type of story they want to read. This page provides a clear visual about the type of tags that are popular on the site, though there are hundreds on canonical tags available. This can help advertise your fic to people who are looking for a specific type of story (AUs, hurt/comfort, fluff, happy ending, slow burn, ect.).

The others have provided pretty good explanations, but I'll add one thing: Use them to express your comment preference! People won't comment if they don't know what you want. You can use tags to express if you're okay with concrit, concrit only, no crit, or whatever else. You should say it in your author's notes, too, but it's good to cover all your bases.

I use "All Comments Welcome", myself, and a friend uses "Unsolicited Concrit Always Welcome".

The way I think of it is as a filter system.

You write down/mention all things in your story that you think might make people interested in reading it or that people might want to avoid. So you might want to mention that there is going to be smut. Or you could mention that its a specific type of AU.

The way I try and see if something is a valid tag before I put it in my story is to check out all the other stories that use that tag. If there's a good variety of work form different fandoms (so it's not so niche) it'll probably be useful to your readers.

Not sure how helpful it is... but this is the way I think of it.

There are warning tags for things such as violence and non-con.etc. That's the main one to focus on and it's pretty simple since there's only a handful.

The other tags are fill in, you can put as many as you want but don't put too much. I find that people who are new to ao3 feel the need to list endless tags so that it pops up in as many places as possible. I find it draws readers away, both from my own personal experience and what I've heard others say. Just focus on the main tropes/genres of your story, what the basis of your story really is. For example: angst, fluff, friends to lovers, canon complaint, fantasy au. You have free reign there.

Same goes for tagging characters, if your story has a plethora of character appearances (such as a crossover) I wouldn't mention every single one and focus on the main players of the story. You really wanna keep from clogging up your tags whenever possible.

You can also (and preferably) go into more detail about the triggering aspects of your story here. If you use one of the warning tags to tag violence, you can also use the fill in tags to get specifc about what this violence is. For example: self harm, murder, gun violence, body horror.

It helps to look at the tags of other stories you may be reading on ao3, so you acquaint yourself with the more commonly used terms.

With regards to archive warnings being mandatory, Author Chose Not To Use Archive Warnings is also an archive warning and a valid option in case you're either not sure if an archive warning applies or you don't want to give away what happens in your fic. ACNTUAW means that any or all of the archive warnings could apply. It's different from No Archive Warnings Apply, which means that none of the warnings feature in the fic. If you put ACNTUAW on your fic, AO3 will consider the archive warning obligation to be fulfilled no matter what.

Good tags: femslash, Angst, Fluff, post-canon, Alternate Universe, Major Character Death

Also good would be adding a new tag that doesn't show up in the drop-down selection (e.g.: "Reina de Champeney" or "Anana the Bright" don't show up in the options of characters to tag, so I wrote their names and tabbed out, and the system took them in each case; once two other people shall have done the same, then the tag wrangler for that fandom will create the official tag for her).

If pairing A/B characters (or A & B characters), do also name their individual tags (apparently the search system doesn't "see" them individually when in the paired form).

If tagging a specific sub-set of a larger fandom (e.g.: MCU), it will show up within the larger-set searches (e.g.: Marvel).

*****

Bad tags: Sorry but my main character, and I really don't know, HEY!, LOL

Also bad would be tagging a popular tag that has nothing to do with the story (this will get spotted though) or has only the slightest technical connection (e.g.: 50K words into the 100K story someone merely mentions an off-screen character named Patrick, and the author tags it for "Patrick Stewart" or "Patrick Starfish" or whatever).

Tag a few relevant characters in the character tags. Such as: protagonists, villain, love interest, maybe relevant minor characters. Tagging too many irrelevant characters can upset people who click on your story expecting said characters to be important.

Tag major relationships. Meaning, if your fic is about romance, tag the relationship(s) it is about. In my experience, it's common to not explicitly tag background ships (there's a dedicated "Minor or Background Relationships" tag for that), so if two people happen to be together in your fic, but their relationship doesn't play any major role, don't tag them. Especially so if they're together in canon, anyway.

Categories. F/F, M/F, M/M, Other, Gen. Again, I personally omly tag what's relevant. If the story is about two girls in love, I tag F/F. Yes, there may also be friendship happening, but the clear focus is on the relationship. If I were writing a fic that has both a plot specifically about friendship, and one about the couple's relationship, I'd tag Gen and F/F. "Other" is a catch-all for relationships that don't fit other categories. For me, that means stuff such as: platonic relationships or relationships between n-b characters.

Freeform tags. The big question. You can leave those entirely blank, and in some fandoms that's common. If I look at the Star Trek Voyager fandom, a lot of fics will only tag two characters and nothing else. Or you can painfully overtag and put 500 tags in there. Also happens. My preferred approach, and what I appreciate as a reader, is to keep it limited to a manageable middle ground.

Meaning: tag a few basic details about your fic. AU? Tag that. Canon-compliant? Tag that. Self-insert, OCs, Fix-It, EWE, Slow Burn? Tag that. Next, what it's about, roughly. Time-travel, War, Jedis, Inappropriate Use Of Elemental Powers, Camp Half-Blood, Hogwarts: put a handfull of things relevant to your story, or ways your story differs from canon, here so people can get a rough idea what to expect.

If I take a lot at some of the recent fics in the PJatO fandom, I can see for instance: "Canon Rewrite, Alternate Universe - Canon Divergence, Fluff, Angst, Good Luke Castellan, Aged-Up Percy Jackson". And already I can get a pretty good idea of what to expect. The point of this is so that people can search for what they're looking for - such as, Canon Rewrites that turn evil Luke Castellan into a good guy - and get it. Think "in five tags or less, what really defines my fic?"

And last: the yikes stuff. Anything that might make people nope out. Because you don't want people to read your fic if they're not into it. It's wasting their time, and you aren't getting any kudos from that. Lose-lose. If your fics has Tentacle Monsters, Implied/Referenced Self-Harm, Canon-Typical Violence, Past Child Abuse, whatever - put that in the tags. To what extent is up to you; I tag major stuff that covers the whole story and put chapter-specific warnings in the author's notes for each chapter.

So, tldr: tag relevant characters and relationships only. Tag genre information (AU, Fix-It, Canon-Compliant), a handful of relevant keywords, and major Gross Stuff.