difference between undef and unknown

[Nicolas Rachinsky]

Hallo,

can somebody tell me, what the difference between validity 'undef'
and validity 'unknown' is?

Like here:
pub   1024D/5B0358A2 1999-03-15 [expires: 2009-07-11]
uid       [ unknown] Werner Koch <wk at gnupg.org>
uid       [  undef ] Werner Koch
uid       [  undef ] Werner Koch <wk at g10code.com>

Thanks,
Nicolas