Implicit sharing between accounts and children
Key Takeaway
If a user owns a child case, contact, or opportunity, or has it shared with them, the user gets Read Only access to its parent account, even when the Account org-wide default is Private. The View All Records and Modify All Records object permissions don't give this access.
Implicit sharing is access that Salesforce grants automatically between an account and its child cases, contacts, and opportunities, in both directions. The system creates and maintains this access itself, so sales teams, service reps, and customers can see related records without an admin building extra sharing for each one.
Implicit sharing, sharing rules, and manual sharing
Implicit sharing is built into the platform. Sharing rules and manual sharing are explicit tools: you set them up on purpose to grant access. Implicit sharing is separate from those tools, but access they grant can trigger it.