Row-level security restricts the rows a reader can query. A Region slicer only changes a report view; a reader can clear it. An RLS role filters the semantic model before report measures run. Define the DAX rule in Manage roles, then use View as to test it.
Put a regional rule on Store, where Region belongs, and keep its relationship to Sales active. Test a permitted region, a role that matches no rows and the unrestricted author view. Multiple roles are additive: a user assigned both East and South roles can see the union, rather than their intersection. Workspace editors aren't the right accounts for testing viewer restrictions after publishing. Role definitions travel with the model, while role membership is assigned in the service.