The equipment model hierarchy shown in the previous section describes the various equipment entities and their relationships in the ISA-95 model. These entities are described in detail as follows:

• Enterprise: An enterprise is an organization that has a definite mission, goals, and objectives. It could represent an entire company, a division within a company, or a functional component of a company dedicated to a product or service. Corporate, divisional, and business unit activities are decided at the enterprise level.
• Site: A site is generally a physical or geographic grouping of a portion of an enterprise. A factory could be a site, an entire complex containing multiple factories, or a single building. The exact determination of a site depends on the size of the enterprise and the organization of the various activities within it.
• Area: An area is a physical, geographical, or logical grouping that's determined by the site. It can contain process cells, production units, production lines, and storage zones.
• Work center: A work center is a facility that consists of equipment in a role-based hierarchy. It performs production, storage, material movement, and any other self-contained activity.
• Production line: A production line is a work center that contains equipment dedicated to the manufacturing of a discrete product or a family of products. A production line contains all of the equipment required to make a discrete product or a component of a product.
• Production unit: A production unit is a work center that contains production equipment that converts, separates, or reacts with one or more feed stocks to produce intermediate or final products.
• Process cell: A process cell is a work center that contains production equipment to carry out a processing step in batch operations.
• Storage zone: A storage zone is a work center that can be a location and/or a piece of equipment dedicated to the storage of materials and/or equipment.
• Work unit: A work unit is an entity in the factory that adds value to a product. The way a piece of equipment is used determines whether it is classified as a work unit. If the device requires a sequence of steps, or specific settings to carry out its production function, it is a work unit. Work units in ISA-95 are differentiated based on whether they are used in batch or continuous operations, discrete manufacturing, or storage. There are three types of work unit:
  • Unit: A unit is a work unit defined for batch and continuous production
  • Work cell: A work cell is a work unit defined for discrete manufacturing
  • Storage unit: A storage unit is a work unit within a storage zone that can be a location and/or a piece of equipment that is dedicated to the storage of materials and/or equipment