There are times when you want to take your input data from a port but you need to specify a File URL. Take a look at the graph below as an example (this is taken from the Web Services / SOAP section).

In this example, the XMLPathReader component needs to get its data from the input port. It's main property dialog requests a "File URL". We can in fact specify the input port through the following dialog:

The dialog allows the user to select a string field from the input port metadata. We only have one field on offer in our example.

When you have finished, notice the field was added to the File URLs list.