If a pathname is converted to a namestring,
the symbols nil and :unspecific
cause the field to be treated as if it were empty.
That is,
both nil and :unspecific
cause the component not to appear in the namestring.
However, when merging a pathname with a set of defaults,
only a nil value for a component
will be replaced with the default for that component,
while a value of :unspecific
will be left alone as if the field were "filled";
see the function merge-pathnames and Merging Pathnames.