User-defined extensions.

Reusable metadata extensions that you create exist in the User Defined Metadata domain. When you copy an object that contains user-defined, reusable metadata extensions, the Copy Wizard copies the extensions to the target repository. If the definition exists in the target repository, the copied extensions become reusable in the target repository. If the definition does not exist in the target repository, the copied extensions become non-reusable.

Vendor-defined extensions.

Reusable extensions that other applications create exist in the appropriate vendor domain. When you copy an object that contains vendor-defined metadata extensions, the Copy Wizard copies the extensions to the target repository and retains their values. If the vendor domain exists in the target repository, the metadata extensions become part of that domain in the target repository. Therefore, you can view them or change the values as you do with the other metadata extensions in that domain.

If the vendor domain does not exist in the target repository, you can cancel the copy or continue in the Copy Wizard. If you continue, the extensions are not available in the target repository. When you install the vendor application, the metadata extensions become available so you can view them and change their values.