The symbol :UNSPECIFIC is not permitted as a pathname component for any slot of any pathname. It is also illegal to pass it as an argument to MAKE-PATHNAME, although it is a legal argument (treated as NIL) to USER-HOMEDIR-PATHNAME.

The only use for :UNSPECIFIC is that it is returned by PATHNAME-DEVICE for LOGICAL-PATHNAMEs, as required by [CLHS-19.3.2.1] Unspecific Components of a Logical Pathname.