With the structure catalog you manage definitions of mechanical structure components and folders.
The Current Drawing tab shows the definitions of components and folders in the current drawing while the External and Favorites tabs show the definitions of components and folders in external drawings. You can perform the following operations.
When you rename a definition, all BOM entries and all instances of it are renamed.
When you delete a definition, all instances of it are deleted.
When an assembly is flagged as a phantom assembly, it is removed from the BOM and the parts and subassemblies nested under it are considered to be parts and subassemblies of the parent assembly of the phantom assembly. You can set the assembly as a phantom in the current drawing only, or as a phantom in all drawings. When you set an assembly as a phantom assembly in the current drawing, other drawings that refer to this assembly as an xref component continue to see it as a regular assembly.
When a component is externalized, AutoCAD Mechanical toolset saves the component along with its views to a new drawing file. In the current drawing, all instances of the component link to the newly created drawing because it is now the source drawing that contains its definition. The instances in the current drawing become external reference (xref) components.
When a component is localized, AutoCAD Mechanical toolset copies the definition of the component from the source drawing to the current drawing. All instances of the component link to the definition in the current drawing and are no longer linked to the external drawing. If the external component being localized is an assembly that contains xref parts or subassemblies, use the localize all option to localize them.
Use the External Drawings tab to explore the definitions of compounds and folders in external drawings. When you select a component or folder and import its definition, the definition of the component is shown in the Current Drawing tab. Subsequently, you can insert instances of the component to model space.