Merging takes a pathname with unfilled components and supplies values for those components from a source of defaults.
If a component's value is nil, that component is considered to be unfilled. If a component's value is any non-nil object, including :unspecific, that component is considered to be filled.
Except as explicitly specified otherwise, for functions that manipulate or inquire about files in the file system, the pathname argument to such a function is merged with *default-pathname-defaults* before accessing the file system (as if by merge-pathnames).
18.104.22.168 Examples of Merging Pathnames