On Wikipedia it is Written: NP and co-NP are also thought to be unequal. If so, then no NP-complete problem can be in co-NP and no co-NP-complete problem can be in NP.
So Subset Sum is NP-complete (given integers find a Subset that sums to 0).
CoNp is, the set of decision problems for which a "no" answer has a deterministic polynomial-time verifier.
What does No mean here in the subset sum Example? I can trivally Test if there is a subset that Doesnt sum to Zero.