The other is about the differences between chapters, stories, series, and collections, and why you should choose carefully which you use for different fics.
I am not affiliated with the Archive in any formal way, and never have been, even as a volunteer, but I do use the archive A LOT both for posting my own stories and reading other peoples', so take my opinion for what it's worth. Here's my take on stories/series et al on AO3:
To decide how to categorize your writing on AO3, you should take into account both what feels good for you as a writer, and what works for your readers, as well. You want people to be able to find what they're looking for easily (so they won't throw their hands up in disgust), you want everything related to be together, you want them to be encouraged to comment/kudos. And some people (like me) who are fandom veterans and download copies of every fic they like for personal archival purposes (I've seen too many beloved fics disappear), and it takes very little effort for us to be happy, too.
A story/work is the basic unit. It has a beginning, middle, and an end. It may be long or short, and it may or may not contain internal subdivisions (chapters). You should be able to read it on its own with no problems, and nothing other than a few sentences explaining the previous works if it's part of a series. It can be a work in progress, but other than that, it should be complete in itself. This is what most people think of when they think of a story.
A story/work should not be a dumping ground for short unrelated pieces.
- If someone likes one of them and then gets excited that "oh, wow, there's 30k words of this story!" they are going to be VERY disappointed when they find out otherwise.
- If someone is looking for one particular trope (say, wing!fic, or curtain!fic, or "Howard Stark's A+ Parenting," or any particular trope) and you tag the story as that because one piece has it, they will read the first section, maybe the second section, and probably give up in disgust because what they've read has nothing to do with whatever they're looking for. By putting short unrelated pieces together, you prevent people from finding (and hence READING) the stories of yours that they actually want to read. Lots of people simply are not willing to wade through all of the different tidbits to find the one tidbit that they want.
- It clogs up the tags, especially if you put pieces of different fandoms in the same "story." Say you have a work/story with three Harry Potter ficlets, two Naruto ficlets, and one Rivers of London ficlet. You tag it with all of those fandoms; it appears in the works list for all of those fandoms. Then you add two more Harry Potter ficlets and a Once Upon a Time ficlet. Every time you do that, it goes to the top of the tag for every fandom it is tagged in. This is especially annoying for the small fandoms, where they don't get much new fic, and they get excited because yay, new fic! only to be disappointed that it is not, in fact, a new fic in their small fandom. They will probably be very annoyed with you and less likely to read your fic in the future because it feels like you are using bait-and-switch tactics. (Yes, I have heard more than one person in more than one small fandom complain about this.)
- You will get fewer readers. Most people choose to read a story based on the summary. They may use tags to find stories they might like, but a large part of the decision on whether or not to read this particular story is the summary. If you have a lot of unrelated short pieces together, you can't really write a summary that has stuff about them all, and hence you are DRAMATICALLY reducing the number of people who will read any of it.
- Sometimes people are looking for a long fic, and will click on a tag (fandom, character, pairing, other tag) of stories they'd like to read and sort by length. Your collection of unrelated ficbits now looks like a long story of the type they want to read! Except it's false advertising, because it's not 50k words of what they want, it's 783 words of what they want with 49k words of other stuff they don't care about. Chances are, they will be very annoyed, and more annoyed if they don't figure out what the deal is right away.
- You will get fewer kudos. If each ficlet is its own thing, and someone reads two of them and likes them, they can kudos both of them. If they are part of the same story/work, they can only make ONE kudos for everything altogether.
- If someone does read all of it, and likes one ficlet but not any of the rest, and they make a habit of downloading their favorite fics to read while commuting or for personal archival purposes, they won't be able to do so.
- There are two exceptions to this. One is for drabbles (true drabbles that are exactly 100 words) and other ultra-short things. Even if you put 50 drabbles together, that's still only 500 words total, and the very smallness mitigates against many of the problems.
- The second exception to this is if you have a series of longfics, and also some really short scenes and gapfillers and outtakes and whatnot. Then you can put them in one story with a summary that tells what it is--"short stories and outtakes from my [whatever] series"--and people won't mind. Everything is there in one place, easy to find (if they like that series) or avoid (if they don't). Everything is clearly labelled and easy to find. It's not clogging up the tags. Even so, if there's something that stands on its own as a story within the series, I would recommend making it its own work, rather than just a chapter in a jumble of scenes.
It is better to have a single work marked as a WIP than to have each chapter of your story posted as a separate work. A chapter is a short piece that does not stand on its own but must be read in order with the other parts of the story.
- Someone looking for a complete work will be very disappointed. They thought they were getting a complete story, and all they're getting is one chapter in a WIP. There are some people who don't read WIPs, only complete fic. If each chapter is a separate work, they can't tell if it's complete or not ... so they won't read it.
- The default at AO3 is to view a fic chapter-by-chapter, but you can also set it to view an entire work all at once. If you post each chapter as its own work, nobody can do this. Those readers who prefer to read a whole work in one page will be annoyed or disappointed.
- Those who download fics to read offline or for archival purposes will find it much more difficult. Instead of downloading one story, they'll have to download a lot and figure out how to get them in the right order on their device.
- It's false advertising. People know what a story is, and they know what a chapter is, and if you give them what is basically a chapter and tell them it is a whole work they will be annoyed. Annoying your readers is counterproductive.
- You clog up the tags. This is especially annoying if you're posting frequently in a medium/small fandom. Someone clicks on that fandom and they get a whole slew of "works" that are, in fact, just different chapters of ONE story. It drowns out other stories and thus annoys other authors and any potential reader who wants to find more than just your fic.
A series is a group of related works/stories in the same plot arc set in a particular order, be it chronological or otherwise. When you put things in a series, you are telling your reader a couple of things. First, that all of these stories belong together quite closely (more closely than a collection) and that they should be read in a particular order.
A collection is a group of works or stories that you believe belong together for whatever reason. Maybe it's "all the fic I've written about Bitty cooking." Or maybe it's "all the fics I've written in any fandom with kidfic." Or maybe it's "all of my favorites." Or maybe it's "all of my tumblr meta ficlets."
Note: I don't think there's much point in creating collections or series with all of your works in a particular fandom in them; it's super-easy for a reader to find them without you doing anything. They click on your username, and get taken to your dashboard with a list of the fandoms you've written in right up there top center. Clicking on the one they want will take them to all of the works you have written in that fandom. On the other hand, it's not like there's any problem with it, or any inconvenience it causes your readers, so it's purely a matter of personal preference.