A user-definable item-item relationship. This item-item relationship does not represent the physical configuration of a parent item with its component item (as is the case with a object breakdown structure), but is the association of one physical item to another physical item. Typical examples of this relationship are:

• Connected To, depicting the physical connection of a pipeline to, for example, a pump.
• Compiler, identifying the compiler application for a software item.
• Test Equipment, identifying the test equipment required for a hardware item.
• Tools, Jigs and Fixtures, identifying the tools, jigs and fixtures required for the fabrication of a hardware item.
• Equipment Schedules, identifying the range of items required for a specific main equipment to perform a specific mission.