Editing the security of the custom object 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.)
Additional security can be configured for each instance with a security override. The system Access Control List (ACL) configured for the definition can only be modified in the administration dialog. Security overrides can be applied to each instance from the properties dialog.
For more on security and security overrides, see Security Overrides.
Perform a security override on a custom object instance
A security override is applied to the custom object instance. This means that the system security is no longer the effective security. The overriden security will be effective until it is removed. At that time, the system ACL will be the effective security.