Operation function is the term used to describe how the core operation of the organization is designed and functions. In manufacturing it is how the raw materials are transformed into a useable product. In sales, it is how the products are received and presented to the purchasing public.
Operation function is important because it is the basis for the business. While it is the basis it is not necessarily the most important but other processes should be subjugated to the operation function. This means the other aspects of the organization must be formed to benefit the operation function. This does not preclude the operation function from benefiting other aspects of the organization.
When implementing the strategy of the organization, the question becomes how to best benefit the operation function to achieve profitability. This means marketing strategies, hiring authority and financial planning must be focused around the operation function. For example, if the operation function is receiving raw wood and building cabinets the other functions need to support that mission. Sales should be designed to focus on the pros of this type of custom cabinetry. The hiring authority should focus on hiring people capable of making the cabinets and financial support should attempt to find cheaper ways to get the materials.
However, the operation function can change over time if needed. Suppose it becomes more profitable to produce tables. The operation function then becomes subordinate to the strategy and must alter its ability from producing cabinets to producing tables.