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.