Here's an example of a TeX question:
This probably should be on the TeX site:
But when I go to flag the article, it only allows me to choose between a few sites.
If it's really clear where it belongs just flag the post for moderator attention and use the
giving the site you think the question belongs on in the message.
Voting to close as "off topic" may result in the question being closed before it could be migrated.
On another question. (I don't know which one) a few mods and Jeff said there can only be five (5) sites that can be on the off-topic list. To move it to another site just select off-topic and then add an comment saying where it should be moved and the mods should move it to the site that you want.