With sharing rules, you are in effect setting automatic extensions to your organization-wide sharing settings for particular sets of users. As shown in the funnel diagram at the beginning of this chapter, this can be considered as opening up visibility and access to records for those users.
Sharing rules can never be stricter than your organization-wide default settings and allow wider data access for the included users or groups of users.
Sharing rules apply to:
Sharing rules extend the access specified by Organization-wide defaults and the role hierarchy. To define sharing rules, navigate to Your Name | Setup | Administration ...