In previous blog, we discussed about Parent Implicit Sharing, let's discuss about Child Implicit Sharing now.
Child Implicit Sharing is ability for Account owner to access child records (contacts, opportunities, and cases), even now owned by Account owner. The account owner's role determines the level of access to child records (read-only or read/write).
The same scenario is also applicable for users above Account owner in the role hierarchy.
Above screenshot taken from org. with all contact, opportunity, and case sharing setting is "Private". If Contact sharing is "Controlled by Parent", you will not see Contact access here - Account owner will have full access of the Contact, even the Contact is owned by other user.
Object (contact, opportunity, and case) with sharing "Public Read/Write" will be also not shown here, because all user have permission to read and edit the records.