Every variable has a customised script which is the only authorised way to retrieves value from messages, gems etc.. AddedValues always returns a default value if the value does not exist in the entity of interest. The default value property can be assigned at will, but it must be of the appropriate type for the variable and obey the Value Must Be constraints, if any.
For a string valued variable, the default value may be any legal macro expression. It must start with { and end with } and will only be evaluated if an attempt is made to retrieve a value and no value exists. It is then evaluated in the context of the current page. Example use {manilaextras.memberAttribute("", "name"))