A user has access to a record, but why? Is it because of their role? Their territory? From a sharing rule? Now you can see why a user has the access they do, right from Record Sharing Hierarchy in Lightning Experience. Previously, you switched to Salesforce Classic to see this information. Sharing Hierarchy is now available on the action menu, not just in the Share window.
Where: This change applies to Lightning Experience in Professional, Enterprise, Performance, Unlimited, and Developer editions
How: Select Sharing Hierarchy from the action menu on the record. Click View next to a user’s name. You see the record access the user has, and the reason for the access in the displayed table.