Please only use H style harnesses that have a back and chest strap.
The ones that dont have a chest strap can still easily hurt your rabbits.
The chest strap allows any pressure put on the harness by an animal pulling to be distributed over the chest rather then the neck. They also sit lower on the neck so same thing. So if someone accidentally tugs on the leash or the rabbit pulls it is safe. Its not cutting into the rabbits abdomen or slipping further down. The chest strap makes both neck and body straps sit in a better place so their on the rabbits properly rather then migrating to the squishy abdomen and squishy neck of a moving rabbit.
It also prevents the rabbit from being able to slip their head under that strap if its just a smidge too loose.
Think of a seatbelt in a car. You want it over sturdy bone parts not the neck or abdomen.
So for a rabbit that is their chest/ribcage.