Final answer:
Trenched boundaries are shoreline types that coincide with subduction zones at plate boundaries, such as the Japan trench, where an oceanic plate subducts beneath another plate.
Step-by-step explanation:
The shoreline that coincides with plate boundaries is called a trenched boundary. Such shorelines occur where a tectonic subduction zone is present, influencing the formation of ocean trenches along coastal areas. These boundaries are marked by significant geographic features such as the Japan trench. Here, the oceanic plate subducts beneath another plate, usually a continental plate, resulting in high pressure and temperature conditions that lead to melting of the subducted plate material. The process is part of the global pattern of crustal creation and destruction, where the amount of crust destroyed at subduction zones is balanced by new crust formed at mid-ocean ridges.