Although what Mr.nobody says is generally true the subtleties to the Bosal are many and the adjustment is for the horse you are riding and the training level of that horse.
An example of different setups would be the diameter and flatness of the nose piece,
The length of the Bosal.
The diameter of the lower part of the Bosal.
The size and weight of the heel knot.
The diameter and weave of the macate reins.
The coarseness of the weave of the rawhide on the different parts of the Bosal.
The material used other than rawhide.
Bosals have lost favor in the modern world because there was so much to know about them.
They are very,very old in design.
There advantage was that before the days of dentistry the horse could be ridden and the mouth that the Spanish revered could also be protected.
Many people that I know will set up a coarser Bosal with more slack to it for the first rides to create more "Bump" and "Release" with the horse that is getting use to the Hackamore.
Later after the cues are refined some the setup is changed.
It is a wonderful training tool and one of the original bit-less setups.
The National Reined Cow Horse Association keeps the tradition alive by offering competition between horses that are ridden in a legal Hackamore.
"The greatest strength is gentleness."
- Iroquois Proverb