About Unplaced Components
Unplaced components belong to an assembly without being assembled or packaged. These components appear in the Model Tree but not in the graphics window, as they are not placed geometrically in the assembly. Unplaced components are indicated by in the Model Tree.
Constrain or package unplaced components by selecting them from the Model Tree, and then right-click and choose Edit Definition from the shortcut menu. After a component is constrained or packaged, it cannot be reverted to unplaced. Unplaced components can be included or excluded when creating a bill of material and are not accounted for in mass properties calculations. When its parent assembly is retrieved into memory, an unplaced component is also retrieved.
You can perform actions on unplaced components that do not involve any knowledge of either the placement in the assembly or its geometry. For instance, you can associate an unplaced component with a layer, but you cannot create a feature on an unplaced component.
When a component is declared to belong to an assembly by modifying its relationship in a PDM system or Pro/INTRALINK, that component is left unplaced in the assembly until it is either explicitly constrained or packaged.