How does object locking affect parent and child objects?

An object is locked only when it is directly modified. Modifying a child object will not result in a lock on the parent object.

For example, consider a Custom Context subtab (a subtab that displays content of a different object), which is added to the Premium Accounts object. The Custom Context subtab displays the Accounts object, which is the parent object of Premium Accounts. The object lock is placed on Premium Accounts and not Accounts, because Accounts (the parent object) was not modified.