Represents an entire STEP file composed of a header and a list of entities.

A Header represents the data stored in the header section of a STEP file:

• description should be an informal description of the contents of the file.
• implementationLevel will typically be "2;1" indicating version 2 of ISO 10303, conformance class 1 (which in turn means that the file has a single data section and no anchor or reference sections, along with several other restrictions). For other possible values, see section 8.2.2 of ISO 10303-21.
• fileName may be the file name of the actual file, or it may be an abstract name for the contents of the file used when cross-referencing between files.
• timeStamp should be an ISO 8601-formatted date and time.
• author should include the name and address of the person who created the file.
• organization should be the organization that the author is associated with.
• One of preprocessorVersion or originatingSystem should identify what CAD program was used to generate the file. This does not seem to be used terribly consistently!
• authorization should include the name and address of whoever authorized sending the file.
• schemaIdentifiers identifies the EXPRESS schema used by entities in the file. This will usually be a list containing a single string, which may be either a simple string like "IFC2X3" or an 'object identifier' such as "AUTOMOTIVE\_DESIGN { 1 0 10303 214 1 1 1 1 }" (more commonly known as AP214).

An Entity represents a single entity stored in the data section of a STEP file. An entity may be a point, a curve, a part, an assembly, or even an entire building. Entities may be 'simple' (having a type and a list of attributes, which can themselves be references to other entities) or 'complex' (effectively a list of simple entities combined together).

An Attribute represents a single attribute of an Entity, such as an X coordinate value, a GUID string, or a reference to another entity.