# Reference Models for Digital Manufacturing Platforms
## 3 Reference Models Alignment
### Reference Model for Industrie 4.0 (RAMI 4.0)
#### Hierarchy Levels  
The third axis of the RAMI 4.0 reference model is the hierarchical representation of the different functional levels of the factory, based on the IEC 62264 [33] and IEC 61512 standards. For a uniform consideration covering different manufacturing system architectures, the different functional levels that are included in the RAMI 4.0 hierarchy levels do not match, on a one-to-one basis, the hierarchical levels in the aforementioned standards. The relationship between the RAMI 4.0 hierarchy levels and the IEC standard organizational models is described below:  
* Connected World: Describes the relationship between an asset or combination of assets and another asset or combination of assets at a different installation or company. This level is introduced in RAMI 4.0.
* Enterprise: Any business organization, initiative, venture, or undertaking with a defined mission. An enterprise is a collection of one or more sites. It is responsible for determining what products will be manufactured, at which sites and, in general, how they will be manufactured.
* Site: A site is a physical, geographical, or logical grouping determined by the enterprise. The main production capability usually identifies a site, and generally, they have well-defined manufacturing capabilities. Planning and scheduling normally involve sites. This level is not included in the RAMI 4.0 hierarchy level, but it is described here to better describe lower levels.
* Area: A physical, geographical, or logical grouping determined by the site. Areas generally have well-defined capabilities and capacities. Areas may contain one or more of the lower-level hierarchical level elements. As with sites, this level is omitted in the RAMI 4.0 hierarchical levels, but it is introduced to clarify the description of the levels below.
* Work Centres: Depending on the manufacturing system organizational model (discrete, batch, continuous), areas are organized in high level manufacturing elements (e.g., production line, storage zone, process cell). In RAMI 4.0, all of these higher-level elements are unified into the work centre concept to ensure a consistent application across different organizational models. Thus, work centres represent the highest level element that performs manufacturing functions and is regarded in production planning and scheduling. Work centres have well-defined manufacturing capabilities and throughput capacities. A work centre groups one or several work units. According to the standard, some examples are Bottling Line, or Assembly Line.
* Work Units or Station: Represent lower level elements that perform manufacturing functions and are regarded in production planning and scheduling. Some examples are work cells for discrete manufacturing processes, or process units for batch manufacturing processes. Work units have well-defined manufacturing capabilities and capacities and are composed of lower level equipment units that are not regarded in production planning and scheduling.
* Control Device: Represents the logical control of field devices.
* Field Device: Represents a device installed at the field level, which interacts physically with the manufacturing process and the products (e.g., a sensor).
* Product: Describes the product to be manufactured.