STEP is a standard for description of data to be exchanged in the industrial world, managed by ISO(TC184/SC4). It is organized in PARTS, which define general organization of data (data format, access mode ..), generic resources (general data definitions, e.g. Geometry, Topology ..), integrated application resources (sets of generic resources that are common to various applicative domains), application protocols (which are focused on specific applicative domains), also application interpreted constructs (which provide additional definitions to complete an application domain). Added to these descriptions, which define data schema itself, in regard of the openness and capability of combinations, recommended practices have been written to help implementors producing best interoperable files.

STEP is very modular in its definition, different application protocols may share many common definitions, hence they can interoperate for what is common between them. For a given applicative domain or set of domains, such as 3D CAD, relevant data definitions are taken from some parts, that we can use as reference. If a new domain is to be considered, additional data and constructs come.

Each STEP Part is identified by a number; groups of numbers correspond to various kinds of Parts. Here are listed the most noticeable STEP parts in use in the domain of mechanical 3D CAD :

• Part 1 : Overview and fundamental principles
• Series 1x : Description Methods
• Part 11 : The EXPRESS language reference manual
• Series 2x : Implementation method
• Part 21 : Clear text encoding of the exchange structure
• …
• Series 4x : Integrated generic resources
• Part 41 : Fundamentals of product description and support
• Part 42 : Geometric and topological representation
• Part 43 : Representation structures
• Part 44 : Product structure configuration
• Part 45 : Materials
• Part 46 : Visual presentation
• Part 47 : Shape variation tolerances
• …
• Series 1xx : Integrated application resources
• Part 101 : Draughting
• Part 109 : Assembly model for products
• …
• Series 2xx : Application Protocols (AP)
• AP 203 : Configuration controlled 3D designs of mechanical parts and assemblies
• AP 214 : Core data for automotive mechanical design processes
• …
• Series 5xx : Application Interpreted Constructs (AIC)
• …
• Series 1xxx : Application Modules (used for definitions of modular protocols)

The protocols AP203 et AP214 are seen more in detail in dedicated pages.