In Vanilla Minecraft, a Cauldron can hold 3 bottles of water, which equals one bucket. This means that bottles can hold 1/3 of the contents of a bucket.
However, jars in Supplementaries hold up to 12 Bottles, or 3 buckets. This means that 1 bottle can hold 1/4 of the contents of a bucket.
This leads to a (small) exploit where you can have a jar flowing into a cauldron to produce an infinite amount of water using a single bucket. (which, I mean, is already possible by using seagrass)
Also, the units used in the tooltip of a jar are mBtl, which I assume are millibottles, 1/1000 of a bottle, but the unit should just be bottles

So basically it's consistent with all containers except for water cauldron

You have to thank forge and past mods for this. They made so 1 bucket is equal to 1000mb which is not divisible by 3. To be fair can't really blame them as this pretty much predates cauldrons. The convention that 1 bottle is 250 mb has been already adopted by all mods and we all agree on it so there are no dupes. All except for cauldron water but that's not a big deal as water can already be duplicated. Also supplementaries usesits own system whosebase unit is not some arbitrary fraction of a bucket but the bottle itself. mBtl is Minecraft bottle. Guess I could remove the m which was just there for consistency with mb (Minecraft bucket) which could also be reads Milli bucket

Yeah, that makes sense. Though Create's fluid tanks (at least on Fabric) are consistent with cauldrons, so I thought I should mention it.

Hm are you sure? Create tanks using mb directly as as I've said I bucket is not divisible by 3. When I made this stuff I tested with create and such and I'm almost certain they use the 250mb per bottle convention

Yeah, filling a tank with one bottle shows it as 333 1/3 mB

Well then that's an inconsistency of its own as if you keep converting you'll loose 0.3... fluid each time. Anyways I got confirmation that on forge create indeed uses the convention of 250. I will not attempt to have a different implementation for fabric and infact for the above mentioned reason I actually think they are wrong. If fabric wanted to have a better fluid system they should have used the bottle as a base unit, say 100 units is 1 bottle so then bucket is 300

I think it's actually measured in droplets, then converted into millibuckets.
There are 81,000 Droplets per bucket, so that makes it easier to divide by 3's. I was also using jade and disabling jade rounds to the nearest whole number.

Oh so they do have their own bottle base system. Sucks a lot for version parity but fortunately I don't have support for fabric flui API yet like I fo with forge so you can't even directly convert. Still don't get what the conversion tho

the conversion is purely cosmetic afaict, it's still 81k Droplets per bucket, it just says 1000 mB. I mean I do prefer 4 bottles per bucket since Minecraft likes powers of 2, but that's just not how it works I guess. Maybe if brewing stands supported 4 bottles at a time...

Anyway, it seems like this is a non-issue so I'm just going to close it