Implicit sharing and role settings for account child records
Key Takeaway
Access to an account's contact, opportunity, or case gives you Read Only access to that account. An account owner's role sets the owner's access to the account's contacts, opportunities, and cases (View, Edit, or No access), whoever owns those child records.
Implicit sharing is built-in Salesforce behavior that links record access between an account and its standard child records: contacts, opportunities, and cases. Access can flow up from a child record to its parent account. Access can also flow down from an account to its child records. Knowing both directions lets you predict who can see an account and its children.
How implicit sharing works
Implicit sharing is built into the platform and works automatically, so you can't turn it on or off.
Implicit sharing applies only to standard relationships, so it doesn't apply to custom objects.