ISO 10303-214:2010(E)
An Item_instance is the occurrence of an object in a product structure that is defined either by a Design_discipline_item_definition or by a Product_identification.
An Item_instance shall be used, at least once, in a Product_structure_relationship with relation_type 'realization' or in an Assembly_component_relationship
EXAMPLE In the case of a car, the Item 'wheel' is defined once. Its Design_discipline_item_definition carries all the information necessary to define the wheel (e.g., its shape) independent of its usage. Additionally, there are 5 Item_instance objects for this wheel since there are 5 equal wheels used in a car: Front-left, front-right, rear-left, rear-right, and spare wheel. Each of these instances may carry additional information such as placement or function.
Each Item_instance is a Single_instance, a Quantified_instance, a Selected_instance, or a Specified_instance.
The data associated with an Item_instance are the following:
The definition specifies the Design_discipline_item_definition or the Product_identification that serves as a definition for this particular occurrence. If an appropriate Design_discipline_item_definition is available, this Design_discipline_item_definition shall be used as definition, otherwise a Product_identification shall be referenced.
See item_instance to design_discipline_item_definition and item_instance to product_identification for the application assertions.
There shall be exactly one object that defines the definition for an Item_instance.
The description specifies additional information about the Item_instance.
See item_instance to multi_language_string for the application assertion.
The description need not be specified for a particular Item_instance.
If present, there shall be exactly one object that defines the description for an Item_instance.
The id specifies the identifier of the Item_instance.
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