Controlled by Parent org-wide defaults
Key Takeaway
With Controlled by Parent, a user can view, edit, or delete a child record only if they can do the same action on its parent.
Controlled by Parent is an org-wide default access level in which a user's access to a child record follows their access to the record it is linked to. Related records such as contacts, activities, and master-detail children stay in step with their parent instead of being shared on their own.
How Controlled by Parent works
Controlled by Parent is one of the access levels you can choose for an org-wide default. With this level, a user can perform an action on a child record only if they can perform that same action on its parent.
For example, imagine a contact linked to the Acme account. A user can edit that contact only if they can also edit the Acme account.