Editing the security of the custom object definition changes the security of each instance created in the Vault.
For example, editing the security for a custom object definition called Contacts would change the security for each instance of a Contact (e.g., Bob Smith, Marla Green, Bakersfield Motors, etc.)
However, if you want to customize the security for a particular custom object instance (e.g., only Bob Smith), use a security override.
Perform a security override on a custom object instance
The security override is applied to the custom object instance.