EXPRESS schemas. This is done by collecting data from several data_sections and referenced instances from other data sections. governing_schema, the EXPRESS schema to which the indicated population belongs to and by which it can be validated. determination_method to figure out which instances belong to the population. Three methods are predefined: SECTION_BOUNDARY, INCLUDE_ALL_COMPATIBLE, and INCLUDE_REFERENCED. governed_sections, the data sections whose entity instances fully belongs to the population. The concept of FILE_POPULATION is very close to schema_instance of SDAI. Unfortunately, during the standardization process, it was not possible to come to an agreement to merge these concepts. Therefore, JSDAI adds further attributes to FILE_POPULATION as intelligent comments to cover all missing information from schema_instance. This is supported for both import and export. SECTION_LANGUAGE allows assignment of a default language for either all or a specific data section. This is needed for those Express schemas that do not provide the capability to specify in which language string attributes of entities such as name and description are given. SECTION_CONTEXT provide the capability to specify additional context information for all or single data sections. This can be used e.g. for STEP-APs to indicate which conformance class is covered by a particular data section. DATA section The DATA section contains application data according to one specific express schema. The encoding of this data follows some simple principles. Instance name: Every entity instance in the exchange structure is given a unique name in the form "#1234". The instance name must consist of a positive number (>0) and is typically smaller than 263. The instance name is only valid locally within the STEP-file. If the same content is exported again from a system the instance names may be different for the same instances. The instance name is also used to reference other entity instances through attribute values or aggregate members. The referenced instance may be defined before or after the current instance. Instances of single entity data types are represented by writing the name of the entity in capital letters and then followed by the attribute values in the defined order within parentheses. See e.g. "#16=PRODUCT(...)" above. Instances of complex entity data types are represented in the STEP file by using either the internal mapping or the external mapping. External mapping has always to be used if the complex entity instance consist of more than one leaf entity. In this case all the single entity instance values are