Also known as: 乳ベルト
Any strap that is worn on top of clothing or that is part of clothing, that crosses a character's chest. This does not include belts with buckles, a belt with a buckle should be tagged as Chest Belt.
If the strap is part of a Chest Harness, just use that tag alone, unless there is a separate strap not connected to the Chest Harness. If the strap crosses the torso diagonally, from above one shoulder to the waist, it is a Shoulder Strap.
If the strap is directly underneath the breasts and touching them, consider adding Underbust. If it is not directly under the breasts, use Belt instead.