External organization-wide defaults are designed for this exact separation of internal and external access. UC can keep the internal default for Delivery at the level required by employees while setting Default External Access to Private for authenticated external users. That creates a restrictive baseline for distributors without reducing internal visibility. Sharing rules are additive: they can open access beyond the organization-wide default, but they cannot take away access already granted by a public OWD. For that reason, neither an owner-based nor a criteria-based rule can be used to make external access more restrictive. After the external OWD is Private, UC can selectively grant the required records through the appropriate Experience Cloud mechanism, such as sharing sets, partner roles, sharing rules, or other supported sharing constructs depending on the relationship model. This follows the least-privilege design principle used throughout Salesforce security architecture: establish the most restrictive baseline that satisfies the broad population, then add narrowly scoped exceptions. Study Guide reference: Access to Records - organization-wide defaults, external OWDs, Experience Cloud record access, sharing rules, and least-privilege sharing.