In addition to sharing setting defined by system admin, there are a number of sharing behaviors that are built into Salesforce applications. This kind of sharing is called implicit sharing, because it is not configured by administrators; it is defined and maintained by the system.
Implicit sharing is automatic. You can neither turn it off, nor turn it on — it is native to the application. In other words, this isn't a configurable setting; however, it's very important to understand.
Parent implicit sharing provide read-only access to parent records (Account only), when user has access to children Opportunities, Cases, or Contacts for that account. This also meant when user have access to record from other object (NOT opportunity, case, or contact) that lookup to an Account, user will see the Account Name only, but not to access Account detail - this include Account lookup to the Parent Account, child account owner will see Parent Account Name only.
The same behavior apply when lookup from other objects, including custom object.
- Implicit Sharing
- Built-in Sharing Behavior
- A Guide to Sharing Architecture
- Performance Tuning for Visualforce and Apex