A boundary diagram, also known as an interface matrix, defines the system that p-diagrams are to analyze as well as surrounding systems. The objectives are to show:

Beside capture failures from system components, boundary diagrams help to capture failures that occur because surrounding systems are functioning improperly. While not within the boundary of the defined system, surrounding systems can physically interface with this system. They can also exchange or transfer energy, material, or data with this system.

To improve system performance, the p-diagram team must include engineers who have the authority to change control method parameters for components within the system boundary. In addition, the team can include people from quality, manufacturing, and testing departments.