Sharing rules are used by administrators to automatically grant users within a given group or role access to records owned by a specific group of users. … All implicitsharing added by Force.com managed sharing cannot be altered directly using the Salesforce user interface, SOAP API, or Apex.
Use sharing rules to extend sharing access to users in public groups or roles. As with role hierarchies, sharing rules can never be stricter than your org-wide default settings. They just allow greater access for particular users. You canshare records owned by certain users or meeting certain criteria.