In Developer Edition, Validation rule for the 'Account Site' field (field of type text) of Account object is, ISCHANGED( Site ) && OwnerId < > $User.Id to make sure that if the 'Account Site' field has been modified and the user is not the owner of the Account record, then an error message would be shown. The System Administrator has Read All, Modify All permission for the Account record. So the System Administrator will be able to modify the 'Account Site' field of the