A key system function is a logical grouping of system functions defined to support the architectural analysis. A key system function could be:

• a group of system functions that are cohesive and/or tightly coupled;
• a group of system functions that may be realized by a single architectural component;
• a group of system functions that will be realized by reuse of an existing component (HW/SW);
• a group of system functions that may be reused within the system;
• a group of system functions that address a specific design constraint

The set of key system functions should include all system functions; however a system function may be member of more than one key system function.