STEP is divided into many parts, grouped into
In total STEP consists of several hundred parts and every year new parts are added or new revisions of older parts are released. This makes STEP the biggest standard within ISO. Each part has its own scope and introduction
The APs are the top parts. They cover a particular application and industry domain and hence are most relevant for users of STEP. Every AP defines one or several Conformance Classes, suitable for a particular kind of product or data exchange scenario. To provide a better understanding of the scope, information requirements and usage scenarios an informative application activity model (AAM) is added to every AP, using "IDEF0
STEP is primarily defining data models using the EXPRESS modeling language. Application data according to a given data model can be exchanged either by a "STEP-File, "STEP-XML or via shared database access using "SDAI.
Every AP defines a top data models to be used for data exchange, called the Application Interpreted Model (AIM) or in the case of a modular AP called Module Interpreted Models (MIM). These interpreted models are constructed by choosing generic objects defined in lower level data models (4x, 5x, 1xx, 5xx) and adding specializations needed for the particular application domain of the AP. The common generic data models are the basis for interoperability between APs for different kinds of industries and life cycle stages.
In APs with several Conformance Classes the top data model is divided into subsets, one for each Conformance Class. The requirements of a conformant STEP application are:
Originally every APs was required to have a companion "Abstract test suite (ATS) (e.g. ATS 303 for AP 203), providing Test Purposes, Verdict Criteria and Abstract Test Cases together with example STEP-Files. But because the development of an ATS was very expensive and inefficient this requirement was dropped and replaced by the requirements to have an informal validation report and recommended practises how to use it. Today the recommended practises are a primary source for those going to implement STEP.
The Application Reference Models (ARM) is the mediator between the AAM and the AIM/MIM. Originally its purpose was only to document high level application objects and the basic relations between them. "IDEF1X diagrams documented the AP of early APs in an informal way. The ARM objects, their attributes and relations are mapped to the AIM so that it is possible to implement an AP. As APs got more and more complex formal methods were needed to document the ARM and so EXPRESS which was originally only developed for the AIM was also used for the ARM. Over time these ARM models got very detailed till to the point that some implementations preferred to use the ARM instead of the formally required AIM/MIM. Today a few APs have ARM based exchange formats standardized outside of ISO TC184/SC4:
There is a bigger overlap between APs because they often need to refer to the same kind of products, product structures, geometry and more. And because APs are developed by different groups of people it was always an issue to ensure interoperability between APs on a higher level. The Application Interpreted Constructs (AIC) solved this problem for common specializations of generic concepts, primarily in the geometric area. To address the problem of harmonizing the ARM models and their mapping to the AIM the STEP modules were introduced. They contain a piece of the ARM, the mapping and a piece of the AIM, called MIM. Modules are built on each other, resulting in an (almost) directed graph with the AP and conformance class modules at the very top. The modular APs are:
The modular editions of AP 209 and 210 are explicit extensions of AP 242.
The STEP APs can be roughly grouped into the three main areas design, manufacturing and life cycle support.
Life cycle support APs:
The AP 221 model is very similar to the ISO 15926-2 model, whereas AP 221 follows the STEP architecture and ISO 15926-2 has a different architecture. They both use ISO-15926-4 as their common reference data library or dictionary of standard instances. A further development of both standards resulted in "Gellish English as general product modeling language that is application domain independent and that is proposed as a work item (NWI) for a new standard.
The original intent of STEP was to publish one integrated data-model for all life cycle aspects. But due to the complexity, different groups of developers and different speed in the development processes, the splitting into several APs was needed. But this splitting made it difficult to ensure that APs are interoperable in overlapping areas. Main areas of harmonization are:
For complex areas it is clear that more than one APs are needed to cover all major aspects: