Scope link
A scope link is primarily used to make packages editable. Simply change the package link to a scope link. As a side effect, you will not be able to receive updates any longer.
Like the package link, the scope link also establishes a new filesystem scope.
The only difference between a scope link and a volume link is that the scope link establishes a new filesystem scope while the volume link does not.
Example
scope - create an empty scoped volume.
Use cases
Debugging packages. Turn a package link into a scope link to allow modifying it.
Creating self-contained environments. Create a scoped volume to use a different version of an installed package within that subtree.