If :unspecific
is the value of a pathname component,
the component is considered to be "absent"
or to "have no meaning"
in the filename being represented by the pathname.
Whether a value of :unspecific is permitted for any component
on any given file system accessible to the implementation
is implementation-defined.
A conforming program must never unconditionally use a
:unspecific as the value of a pathname component because
such a value is not guaranteed to be permissible in all implementations.
However, a conforming program can, if it is careful,
successfully manipulate user-supplied data
which contains or refers to non-portable pathname components.
And certainly a conforming program should be prepared for the
possibility that any components of a pathname could be :unspecific.
When reading_1 the value of any pathname component,
conforming programs should be prepared for the value to be :unspecific.
When writing_1 the value of any pathname component,
the consequences are undefined if :unspecific is given
for a pathname in a file system for which it does not make sense.