This is a great reference:
I think you meant bosal, not dorsal (auto-correct fail?). They are most similar to a hackamore, made of smooth braided rawhide and exert pressure on the horses nose and face when the reins are picked up. They are used with a heavier mane-hair rope rein called a mecate and the bosal itself must be well-fitted to the horse or it will cause rubs. For that reason, they aren’t recommended for beginners unless you have a good trainer who can show you how to shape the bosal correctly and buy one that fits your horse. When fitted properly they are gentle.
A loose ring or D ring smooth snaffle with a bean in the center is about as mild as you can get for a bit. They are a standard first step for a horse with unknown preferences, or starting a green horse. Some horses prefer a single joint instead. Do not use full cheeks without bit keepers.
Any bit with leverage, shanks, or a chain is NOT a snaffle. A Jointed curb bit can be very severe and is often believed to be mild because of the joint. The joint does not make it a snaffle.
Hackamores can be just as severe as any bit depending on the design. They work off of leverage on the nose and under the chin.
Sidepulls have no leverage and are milder. They can be made harsher by using thinner, harder material on the noseband, like stiff nylon rope.