IFC Classes
Every object-oriented data schema is based on classes (entities). The IFC schema defines classes and their dependencies for most physical objects in BIM projects, but also more abstract concepts in the entire lifecycle, like tasks or resources.
When it comes to physical objects, IFC classes are similar to Revit categories, as they define the relationships and properties of every element. Depending on the classification, each element has defined relationships to other elements and predefined property sets according to the model view definition used. If a building element is created using the wrong Revit category and/or exported using the wrong IFC class, it will lack important information.
A wall "knows" that it needs to carry information about its height, width, layer thickness and materials, that it can host other elements like walls and windows, and that it depends on the base / top levels. A generic element created using a 3D extrusion won't have these dependencies and won't be able to export standard wall data defined by the IFC schema automatically.
Revit supports all main IFC classes which can be represented in Revit. A current list can be accessed through the Revit help:
In addition to classes, the IFC schema further categorizes elements into types, similar to subcategories in Revit.
Types are documented in the buildingSMART documentation under Type Enumeration and are displayed in block letters.
Example below: IfcWall in IFC4 RV can have following types: MOVABLE, PARAPET, PARTITIONING, PLUMBINGWALL, SHEAR, STANDARD, ELEMENTEDWALL, USERDEFINED, NOTDEFINED.
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