...
CO: The new value for the foreign key must point to a record that is in the same CO. This is typically the correct setting.
As of Registry v4.0.1, this setting is no longer required. All foreign keys are automatically constrained to the current CO.
yes: Any value is permitted. This should only be used if the foreign key controls a cross-tenant configuration, or if the attribute name for some reason looks like a foreign key, but actually isn't.
...