The WWP does not use the fill() keyword at all.
If we did (or if we do again), this is how it works:
The fill keyword indicates a character string (usually a string of periods and spaces) which is repeated between two elements, usually pairs of items in a list which appear arranged in columns. The WWP does not specify what the character string is, so the argument is left empty. The fill keyword would be used on the second of the two elements, as in the example.