To place parts, select a service and a group.
Video: Creating an MEP Fabrication Model in Revit
When you click the arrow, a drop-down list displays additional parts based on the size. This drop-down list is controlled by the conditions in the service definition, similar to routing preferences. For example, a bend may use a pressed bend up to a certain size, and a segmented bend above that size. When you place the cursor over a part in the list, a tooltip displays the size range based on the condition for that part.
This indicates that the fabrication part is not currently supported with item content (ITM) in Revit. Currently, fabrication components for flex duct, structural members, air terminals, and mechanical equipment are not supported. In place of these components, you can use Revit Family (RFA) based components using the category's standard placement tools.