Not only is this allowed but it's something i encourage all writers of any kind to play with! :D
The idea that all writers know what to say all the time and just splash fully-formed drafts out one word after the other is false. There are some who can do it, but i think most of us... can't. Which is why we need tricks like square bracket notes! They're not cheats or lazy writing or some other flavour of Not Allowed, but instead really really important tools that we should use as much as we need to.
Some of the most helpful tricks I've collected over the years are:
make some notes in square brackets – e.g., I had to write a scene on a sailboat, but I know nothing about sailing so i literally just had notes like [boat part] and [how to do X thing?]. If you use square brackets as punctuation anyway, use something else like [[double square brackets]] or a unique letter combination like XY at the start of the note; the point is to pick something you can search for easily later on.
(You can also style inline comments in a different font/colour. Scrivener has an inline annotation feature; if you use Word, you can make a specific Style to make notes stand out at a glance, etc.)
bullet-point your way through any tricky parts – this can be pure stream-of-consciousness vague ideas. it only needs to make sense to me later. much more helpful than just leaving big blank gaps that Future Me has to work out how to fill, but also better than dwelling on a piece of writing forever.
use comment tools – mostly do this if I have ideas for alternate events and/or phrasing, or if I want to check something for continuity purposes.
write out of order – Best advice i ever got for academic writing is to know or even write your conclusion first and your introduction last, which your main argument in between. Similar principles apply in fiction, or any kind of creative writing. If there's a part of the essay that I can visualise clearly or a part of the story that is particularly exciting or important, I might write that first, then figure out how it fits/how everything fits around it.
keep a loose scenes and/or "outtakes" folder – anything that i write out of order goes here, along with any notes for how I think I want to incorporate it into the full text. In the same vein, if I delete something but don't know for sure it will never be relevent ever again, it gets cut and pasted into an outtakes folder.
Basic rule though is that you do not have to get your writing perfect on the first try. This is where drafts come in. The way I see it is to treat each draft as a fresh start – I create/open a new document (well, new Scrivener file) and start over as if from scratch. Each draft gets a narrower focus than the last. This is my process, as an example:
first draft is the word vomit. You do whatever you need to do to get it onto the page, and it can be terrible. In fact, it probably should be terrible. You can fix everything later. it's fine.
The second draft is a half-hearted cleanup attempt. I'll re-type everything because everything is subject to change, from the characters' personalities to the pacing to the order of events. It's all primordial goop, basically. i'm just poking and prodding and making a few adjustments, but mostly trying to create a more stable version of the first draft. All shortcut tricks continue to be my best friend.
By draft three I'll let myself copy-paste between documents if I'm particularly happy with a passage, but try not to get hung up on anything specific. I'll still make liberal use of square brackets etc. as I need to, but try to address as many from the previous draft as I can. This is where I get more brutal with making decisions and trying to fix parts of the story in place.
Draft four is usually my final draft, but there's literally no rules about how many drafts you're allowed to write. It's at this point that I try to keep square brackets etc. to a minimum (unless i've diverged significantly from the plot of a previous draft and having to rewrite large chunks), and make sure to address all the notes and problems encountered in previous drafts.
This is when I move on to revisions. Revisions are the "final do-overs", for me. I start them when I'm satisfied with all the large-scale aspects: plot and chronology; characters' personalities, motivations and arcs; large-scale pacing (so the over-arcing pace, rather than the pacing in individual scenes); backstories; and worldbuilding. I'll copy the last draft's document instead of starting with a blank one. First I run through those large scale things one more time and tweak until I'm happy, not just satisfied. Then I shrink my focus to in-scene pacing, dialogue, and the quality of the writing itself.
I'll also rewrite my plot outline between each draft, too. The act of actually re-writing stuff is very helpful for making your brain think about it.
Drafting like this isn't for everyone, but realising that you can just bullshit your way through chunks of text was a massive game-changer for me. Some people will do a draft, then work on something else, then come back and do another draft, work on something else, etc. Some people's drafting process will look more like what I consider to be revisions. Do whatever works for you. Just remember that from the moment you first decide you Want to Write a Thing to the moment you hit "post" or "publish" or give your manuscript over to a publisher, you can keep making as many changes as you like in any way you like. (And if you go the querying to traditional publishing route, you'll probably get suggestions for, and have space/time to make, changes to the manuscript quite far into the process).
