The question is travel related: I am wondering about this for carry on.
The most likely answer is: whatever the check-in employees think. For example there's a well-known video by Jaktogo.com where a man succesfully transforms a bag into a 'coat', allowing him to bring it on-board (on Ryanair!):
If you can convince the airline employees that your 'bag' is a 'vest', you're good to go. If you can't, you'll have to pay extra or check it in as a bag.