Universal Containers has a Private sharing model for Accounts and Opportunities and uses Territory Management...

Salesforce provides parent implicit access between certain standard objects. When a user receives access to a child Opportunity, Case, or Contact through ownership or sharing, Salesforce automatically provides that user with Read Only access to the associated parent Account. Sales Rep B therefore receives access to the Account because Rep B was manually granted access to the related Opportunity. No separate Account sharing operation is required. Salesforce specifically states that access to a child Opportunity can grant implicit Read Only access to its parent Account even when Account organization-wide defaults are Private. Territory membership is therefore not required for this particular access path. An Account Team could independently provide access, but the scenario does not state Rep B was added to one. The documented parent implicit-access mechanism directly explains the behavior. Salesforce

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