Well truthfully - no after reading this I was even more lost and had not a clue what you were referring to or what this was for.
So I installed it then sill didn't get it and went to the bash readme and saw the whole section on http://wryemusings.com/Wrye%20Bash.html#BainConversionFiles and then understood.
So then trying to answer my own question: This is basically a file that will repackage the the PRE-defined packages into a single package.
While a person can make their own at anytime they would then be creating their own BCF which then would also be another predefined BCF.
... I never got into the BCF thing because it is not addressing a basic step I consider vital to creating your own packages, which is taking the time to clean the esps before repackaging.
I've not really seen anyone utilize this function and forgot all about it. To me it really is more of a hassle because it still clutters up the Bash Installers folder and actually adds to the clutter. I've around 150 BAIN packages many-most with up to 10 subpackages (sometimes many more, sometimes just simple packages). An advantage of packaging them elsewhere is that if keeps that potentially very messy folder much more tidy.
I guess Wrye saw his BAIN folder as a work desk with project and packages, some in varieties of completion and contents.
But this is not ideal for power mod using (at least not to me). I like creating these packages outside of that folder - otherwise it would very very confusing what is in there.
That is unless I'm missing something very essential.
Yeah, I'm terrible at explaining. Sorry.
Basically it works like this- if you want a compilation similar to my weapons archive, but don't want to or can't do all the moving around and rearranging and work that goes into it, the creator of the archive can release a BCF. You then download the BCF, the necessary archives, apply the BCF and remove the BCF and the source archives. You then have the desired complex package, without any remanining clutter or packages lying around your BAIN folder- yes, you do need to temporarily have the source archives in your BAIN folder. However, once you have applied the BCF, you can delete them. And since applying a BCF takes no more than a minute or so, you're not really messying up your BAIN folder.
If you want to create a BCF for a package that contains a cleaned esp, the cleaned esp is packed into the BCF- so you can do that as well. Of course, there's no real benefit for the author of the BCF- just for the user that would like to have a certain BAIN project (for example, one shown in this thread) but is intimidated by all the work necessary to get it right.
For example, I would, eventually, like to have a Darnified UI BAIN package like yours- however, I am lazy and if it means doing much more work than selecting the install options in the OMOD installer, I can't be bothered. So, if you, hypothetically, created a BCF for your Darnififed UI BAIN structure, I would download all the packages you used to create it, then apply the BCF and delete or move the source archives outside of the BAIN folder. I now have the Darnified UI archive without doing any work at all. Win for me, no gain for you- other than being able to easily share your BAIN archives without permission difficulties.
As far as creating the BCF goes, it works in much the same way that you apply one. You have an archive that you want to create a BCF for so you can share it with others (this is the "target archive") and you have the archives whose files went into the creation of that archive (the "source archives"). In Wrye Bash, you select all the source archives, select "Create Conversion", pick the target archive, and Wrye Bash does the rest. The BCF is now in your Bain Converters folder and you can upload it for the enjoyment of others. Again, if there are files in the target archive not present in the source archives (like a cleaned esp for example), Wrye Bash adds that file to the BCF, and when the BCF is applied by the end user, the file is drawn from the BCF archive instead of one of the source archives.
As another example, say you have a quest mods archive (that you created some time ago) that you would like to share with others. Clearly, it would be endless work to get permission to upload the archive (and many mod authors will be opposed to the idea, so you would have to remove those mods and your archive would be incomplete), not to mention it is a huge file. You've also cleaned all the esp's to make sure you have a completely stable game. You then create a BCF. You get together all of the original mod archives that the mod author created and put those in your Bash Installers folder. In Wrye Bash, you select these source archives, right click on them and select "Conversion", then "Create". Wrye Bash will ask you to select a target archive. This is your finished quest mods BAIN collection. You select it in the file browser and Wrye Bash creates your BCF. This will now contain several items. First is the information of what the BCF is trying to do, what its source archives are, what the target archive is supposed to look like, details about the packages in general etc. Since you also cleaned the esps though, and those files are missing from the source archives, these cleaned esps are also present in the BCF (which is just a regular 7-zip archive).
So you're pretty much done with the BCF now and can delete or remove the source archives from your BAIN folder and upload the BCF. Now, someone sees your compilation in this thread, and thinks to himself "Boy, this looks awesome, but darn, that is a helluva lot of work." So now he sees that you've created a BCF to make his life easier, and excitedly downloads your prepared BCF. He reads the readme, and downloads all the source archives - but omits one because he simply can't stand that mod. He dumps all these archives into his Bash Installers folder, puts the BCF in the Bain Converters folder and opens Wrye Bash. He selects all the source archives, right clicks and selects "Conversions" then "Apply" and finally the BCF he wants to apply. Wrye Bash asks him to name the new archive it is about to create, and then does its magic. It will copy the necessary files from the source archive to the new archive except for the missing one, which it simply omits, and then copy over the cleaned esps because those were not present in the source archives. The user now has the full BAIN archive, except for the one mod which he chose to omit. He can now remove the source archives from his Bash Installers folder, removing any unneeded clutter and install and play with the BAIN archive without having done any work.