A belt that is worn over the chest, either on top of clothing or part of the clothing. A buckle is required; a strap with no buckle is a chest strap.
If the belt is part of a chest harness, use that tag alone. If the belt crosses the torso diagonally, from above one shoulder to the waist, it is a shoulder belt.
If the belt is directly underneath the breasts/pectorals and touching them, consider adding underbust. If it is not directly under them, use belt instead.
Use belt bra if the belt is worn over the breasts like a bra.
