Jerl said:
Mass tagging started to retag all images tagged facesitting into sitting_on_face for the correct tag alias. Please stand by...
In the meanwhile, I'll explain to you exactly why we use the tag the way we do.
You see, the vast majority of images on Gelbooru come in straight from Danbooru. Approximately 75%, actually. And, with their images, we also get their image translations and, more importantly, their tags.
All of the tags that are applied on Danbooru are also applied on Gelbooru. While Danbooru has an aliasing system that is relatively easy to keep up-to-date, Gelbooru's alias system has some problems. For example, images that come in from the danbooru bot, which directly inserts them into the database instead of using the normal post upload procedure, doesn't directly get affected by it, and the tag remains as it was. Additionally, when new aliases are added, our search listing must be rebuilt for it to work. On top of this, aliases cannot be broken in any easy manner, and only two people on the site are currently capable of adding aliases. For these reasons combined, we generally do not use our alias system unless absolutely necessary, and instead perform soft aliases with the moderator-only mass tagging tool.
Now, to start with, lots of the tags on Gelbooru started here before Danbooru had any alias set up for them. Additionally, Gelbooru has been around for long enough for many of our images not to have been affected at all by Danbooru's aliases. I don't know when Danbooru implemented aliases, but I do know that I have seen quite a few tags on images that came in from Danbooru that were not aliased on Danbooru.
However, the danbooru scraper is still constantly running, pulling posts straight from Danbooru over to Gelbooru, tags and all. For this reason, we cannot just use soft aliases to fix the new images to fit the new tags. First of all, often the new tag has many more images than the old tag did. Other than that, though, the images with the new tag would just keep coming in even if we soft-aliased them with the mass tagger, which would require us to run thousands of mass tag edits every day. Running the mass tagger once is probably equivalent to about three or more people browsing the site normally. Depending on the tag, this may last from 10 minutes for a tag with about 200 posts to eight hours for a tag with ten thousand. Performing mass tag edits on this scale would eat up all of the site's resources and bandwidth, and would cause more trouble than it's worth.
So, instead, we soft-alias them to what Danbooru uses. This means that we only need to re-perform the mass edits very infrequently. The number of edits that must be performed in these mass edits is usually very small, even if we only perform them every few weeks. For example, I frequently mass implicate animated_gif -> animated. I've let it sit for several weeks and only found about 10 images missing the tag. This is much more efficient for tagging purposes.
So, when Danbooru sets tagging policy, we are almost always locked in to following it ourselves. There are some exceptions, but generally our tagging policy mirrors theirs.
You'll have to look to Danbooru for the reasoning behind their tag aliases. A quick search of their forums found me this thread:
danbooru.me/forum/show/38771I barely looked at the thread, though, so I don't know how relevant it is. You may also wish to look at their tag wiki.
TL;DR we use danbo's tags/rules, and our alias system sucks dick.