The default for a new site is to only have meta as a migration targets. As a community, we can add up to four more by agreeing here on meta what are the most common sites to migrate to.
Moderators can migrate to any site, so if it really needs to be migrated, you can custom-flag for moderator attention - but you should still vote-to-close as off-topic, in case your flag gets declined. Moderators will be applying these three rules before deciding to migrate:
Don't migrate crap. If the question is low-quality, or would need substantial editing to be acceptable to the target site, then just close it.
Don't migrate unless you're sure the question is suitable for the target site. The "migration rejected" experience is pretty bad for questioners. If you are not a close-voter on the target site, check with them before migrating.
There's no point in migrating unless information would be lost. We'll chiefly migrate if there is good discussion in the comments or good answers, because those will be moved with the question. If there aren't, it's not often worth bothering unless the question is very high-effort.