19.2.2.10 Relation between component values NIL and :UNSPECIFIC

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.