The „sharing tab“ on standard Plone content assigns different permissions in different areas to different people or groups.
On the local role form, you can search for another user and assign user roles. You can also assign roles to groups. Most commonly,  you will give other users either the owner or manager role over your content for modification purposes, but with custom permissions and roles, you may have other roles to grant.

Currently, local roles can be added at a lower level in the acqusition tree, but not taken away. That is, if you give a user manager permissions at /publications, there is no way to prevent the other person from having the manager permission at /publications/2006.