What objects cannot be included in a sharing set due to their sharing settings?

The correct choice highlights that objects with an organization-wide sharing setting of Public Read/Write cannot be included in a sharing set due to their inherent sharing model. In Salesforce, a sharing set allows for users in a community (such as partners or customers) to access specific records based on their relationship with the Account or Contact that is associated with their user profile.

When an object has a sharing setting of Public Read/Write, it means that all users have full access to all records of that object, regardless of their relationship with those records. Therefore, it does not make sense to utilize a sharing set for such objects, as sharing sets are designed to grant access to specific records to limited users based on defined relationships.

In contrast, objects with Private or Public Read Only settings typically require sharing rules to grant access to certain users, thereby aligning well with sharing sets. Additionally, the nature of lookup relationships with Account or Contact does not inherently disqualify those custom objects from being included, since their sharing settings can still govern access properly when creating a sharing set.