The predicate term is distributed if the proposition is negative (E or O) and undistributed if the proposition is affirmative (A or I).

When we say that no S is P, we are excluding all Ps from the class of Ss just as much as we are excluding all Ss from the class of Ps. So in this case P is distributed.

In "Some S are not P," the subject term is undistributed, as we saw, because we are talking only about some members of S, not all of them. However, those members are being excluded from the class of Ps in the same way that, in the E proposition, all the members were being excluded. In other words, there is a subset of S that is excluded from each and every P. Thus, we know something about that whole subset of S -- namely, that it is excluded from P.

For the predicate term, what matters is quality. Quantity is irrelevant.