A Public Sector Organization (PSO) is already using Grants Management from Public Sector and has...

A Sharing Set is the appropriate Experience Cloud sharing mechanism when external users need access to records associated with their account or contact but don't own those records themselves.

Salesforce states that a sharing set grants Experience Cloud site users access to records by mapping the user's Account or Contact relationship to a corresponding Account or Contact lookup on the target record. Salesforce gives Cases as a specific example: administrators can grant site users access to all Cases associated with the Account identified on the user's Contact record.

That matches this scenario precisely. The internal support team creates the Case, so the external constituent isn't necessarily its owner. A Sharing Set can expose the appropriate Cases to the authenticated external user based on the associated Account or Contact while keeping unrelated constituent Cases inaccessible.

Changing Case OWD to Public Read/Write would be excessively permissive and violate least-privilege principles. A permission set provides object- and field-level authorization but doesn't by itself grant access to specific records restricted by the Salesforce sharing model. Share Groups solve the opposite direction of high-volume sharing: they primarily give internal users access to records owned by high-volume external site users.

Study Guide Reference: Agentforce Public Sector Implementation Strategies and Best Practices – Experience Cloud sharing; Sharing Sets; Cases; Account/Contact access mappings; external OWD and least-privilege record access.