Like form, function is a system attribute conceived by the architect. It is the actions, activities, operations, and transformations that cause or contribute to performance. Function is made up of a process, which is pure activity, acting on an operand, which is an object changed by the process.

Built systems have a value-related operand, whose change is associated with the delivery of benefit and eventually value. The value of a system occurs when the externally delivered function acts on an external operand across the system boundary.

Externally delivered value-related function and other, secondary value-related functions emerge from the internal functional architecture, which usually features a value-related pathway. A complete description of functional architecture includes internal processes and internal ­operands.

A summary of features that contrast form and function appears in Table 5.6. In the next ­chapter, we will present the core idea of system architecture: the allocation of physical/­informational ­function to elements of form.