Also control can be defined as ""that function of the system that adjusts operations as needed to achieve the plan, or to maintain variations from system objectives within allowable limits"". The control subsystem functions in close harmony with the operating system. The degree to which they interact depends on the nature of the operating system and its objectives. Stability concerns a system's ability to maintain a pattern of output without wide fluctuations. Rapidity of response pertains to the speed with which a system can correct variations and return to expected output.