This binding can be used for Input Option Values or the Default Value like any other binding. However, it is really only useful when used for the Default Value.
When this binding is used as a Default Value for a TV, if no content is specified for this TV in the current document, the binding will look at it’s parent document for content, and if none is found, it’s parent and so on until it reaches the top of the document tree. If it no content is found it will use the default content.