Sharing rules let you make automatic exceptions to sharing settings defined by user profiles based on a record’s field values - to give users access to records that they don’t own or can’t normally see in accordance with the non-owner access level defined in their Profile or their place in the role hierarchy.
Sharing rules, like role hierarchies, are only used to give additional users access to records - they can’t be stricter than your profile settings or global sharing settings.
Sharing rules can be used to grant owner-level access to users of a Profile, or view-only access. You can define one sharing rule for each of these two access levels.
A sharing rule is defined by a formula using Mapsly formula builder, and can contain one or multiple logical conditions, including nested ones, linked by AND/OR operators. Each condition is a comparison of a record’s field value with a constant operand.