A shield is usually a steel structure able to withstand a ground collapse and protect those inside; it may be fixed or portable and moved along as work proceeds. Trench shields and boxes are built for the earth pressures of particular depths and ground types and have designated lifting points. Unlike shoring, which prevents collapse, shields and boxes only protect workers if a collapse happens. They suit places where other support is not reasonably practicable and are mostly used in open areas where an excavator or backhoe can lift them in and out, but not where access or ground conditions rule out lifting equipment. Boxes may be light or heavy duty depending on depth and ground, and a competent person such as an engineer should design them, with manufacture to job-specific dimensions.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.