Recently, I'd suggested a post be migrated. It was denied because the post failed to meet 'migration quality requirements'. Where are those defined? I have read https://stackoverflow.blog/2012/03/22/respect-the-community-your-own-and-others/ which does not seem to spell out much beyond Don’t migrate poorly-asked or non-constructive questions.
TL;DR: Don't send bad questions to other sites.
That's about it. Each site has its own rules, its own preferences, and its own standards. It's important that any question you send to that site meets all of those requirements. If you don't know, won't fix, or don't care about that other site, don't migrate the question. Anything else is tantamount vandalism.
Take the following example.
I hate Mint
I installed it because a friend told me to but it's awful. lol.
That is off-topic here, so that means it's on-topic on U&L or Superuser, right? Would you migrate it? That is obviously a flagrant example, but you do occasionally see people forgetting this and trying to migrate some really awful posts.
The threshold for a bad question waivers from person to person, site to site. There isn't a checklist of things I can say will definitely mean a question is accepted. Most people reserve them for objectively good questions in favour of just closing as off-topic and suggesting another site. Migrations are fairly rare.