This is now live. Use this option when…
- a question is clearly out of scope (e.g. about a web app), and
- it is too bad to be migrated to another site, or there is no target site for this question
Being more specific never hurts though, so if you choose Other… and fill out a custom reason, that's fine as well.
As for the one slot we can fill, we'll take suggestions here, or in the form of separate Meta posts.