Extra Property is a geometric parameter by which two similar elements differ with all other properties being the same. In most cases, these parameters affect cost in a cost estimation report.
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Open the Breakdown Manager and you will see that your model has a traditional Revit Breakdown structure (Category - Family - ElementType) by default. Extra Properties are placed right after ElementType.
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Eg. One parameter by which two types of ceilings differ is the thickness (20 mm for the first group and 30 mm for the second one).