[Rock-dev] Semantic difference between base::unknown and base::NaN?

[Arne Böckmann]

Moin,

base::unknown and base::NaN are both defined as quiet_NaN. Is there a 
semantic difference between the two?
If yes, what is the difference? When should I use base::NaN and when 
should I use base::unknown?
If not, why does base::unknown exist?

Cheers,

Arne