Given that the information about the creation of a site-specific close reason isn't very well-documented, I figured I would provide a broad explanation about what they are, how they can be created, and when we should create them.
Only create one if you really need one
First, it's important to note that you should only create a site-specific close reason if you frequently run into a situation where you need it. If you observe that you're experiencing a series of questions that aren't fit for your community that you're closing with a consistent "Other" close reason, or you're creating some new policies that make certain types of questions off-topic that were previously acceptable... Then that's a great place to consider the creation of a site-specific close reason.
It's important to note though that new sites have a very limited number of site-specific close reason "slots" available: Three. This can be raised up to five with Community Manager approval, but you should be prepared have a really good reason to raise that ceiling. I do not, currently, see a reason for us to have more than even one on this site, but that may quickly change in the near future.
Drafting and creating a close reason
To create a new close reason, the community needs to come together and determine how it should look. This should, typically, be done with a Q&A about each close reason so you guys have a dedicated space to discuss each one in detail and nitpick to your hearts' content. Talking about them in this kind of list-style Q&A is good to get a general feel for how much community buy-in there is for a new one, but the actual proposal should be its own Q&A.
Catija explained the different fields that need to be filled out when creating a site-specific close reason, and I followed that process for one of our close reasons on Meta SE. One important note is that no community manager intervention is required when creating a new close reason, it's only required when you just want to updating an existing one. Even in the latter case, moderators can choose to retire and replace an existing close reason with a new one and bypass that requirement (but that should only be done if the close reason is sufficiently different upon revision). So, when y'all are ready for one, you only need two moderators: One to propose a new close reason, and one to approve it.
Now then! There are five fields that are needed to create a new close reason. I'm going to paste my explanation for each field from my Q&A here, which is based on Catija's answer here:
The format is as follows:
- Brief description (100 characters) - Visible in the close/flag dialog in bold. Should be very succinct, only spanning a couple of words.
- Usage guidance (500 characters) - Tells close voters when to use the close reason. Ideally, this is descriptive enough to cover edge cases, and that detail should inspire confidence in close voters that this reason is the right choice under specified circumstances.
- Post notice close description (500 characters) - Visible to all users, the post notice description should follow the format "This question was closed because it is ... It is not currently accepting answers."
- Post owner guidance (500 characters) - Visible only to the post author, this should explain to the OP why their question was closed, and give detailed information on how to improve their post to get it reopened. Helpful links to the help center would be nice to include.
- Privileged user guidance (500 characters) - Visible to users with the cast close votes privilege in the post notice on a post closed with this close reason. This should inform close voters on how best to guide the asker in improving their question or inform them when the question should be reopened.
Once you guys have a draft that seems to fit the bill for the kind of close reason you're trying to create, have a moderator propose the close reason and another moderator approve it.
For mods, the URL where this is handled will be: https://langdev.stackexchange.com/admin/flags/site-specific-close-reasons
For users with the access to moderator tools privilege (aka 10k tools), you can navigate to https://langdev.stackexchange.com/tools/question-close-stats to have a look at all of the question close reasons and their related stats.