Planning theory illustrates
The open-system approach to management.(comment)

The system:
The systems approach considers two basic components: elements and processes. ELEMENTS are measurable things that can be linked together. They are also called objects, events, patterns, or structures. PROCESSES change elements from one form to another. They may also be called activities, relations, or functions. In a system the elements or processes are grouped in order to reduce the complexity of the system for conceptual or applied purposes. Depending on the system's design, groups and the interfaces between groups can be either elements or processes.
Because elements or processes are grouped, there is variation within each group. Understanding the nature of this variation is central to the application of systems theory to problem-solving.
Open vs Closed Systems:
Some systems are open with respect to certain elements or processes. The elements or processes can flow into or out of the system. For example, an automobile engine is “open” with respect to gasoline--gasoline flows in and exhausts (oxidized gasoline) flows out.
Other systems are closed with respect to certain elements or processes. The elements or processes do not leave the system. For example, an automobile engine is largely "closed" with respect to lubricating oil--the oil does not leave the engine.
Planning takes in to account the open system approach

Planning theory illustrates the open-system approach to management