In addition to permissions for owners and non-owners of records (described in detail in the Profile & permissions article), you may define up to two record sharing rules, each consisting of a condition and the target access level: when a record meets the condition of a rule, the user’s access level will be elevated based on the specified access level, which can be set to Hide, View, Modify, or defined for each field separately.
By elevating we mean that:
If the user didn't yet have access to the record, they will receive access.
If the user had Hide access level for a field based on their owner/non-owner permissions and the rule granted them View access, the user will have the View access. But if a user already had Modify access to a field, the access level will remain Modify and will not lowered to View.
Elevation of access level is performed on a per-field basis. So if a records meets the conditions of both sharing rules simultaneously, Mapsly will apply the higher access level for each of the fields based on both rules (in combination with the owner/non-owner permission sets).
By design, record sharing rules can only be used to raise the access level (grant additional permissions) to records, and cannot be used to restrict access.