Two propositions with the same subject and predicate terms but differing in quality (both particular) are

Two particular propositions with the same terms differing only in quality are sub-contraries.

The I form 'Some S is P' and the O form 'Some S is not P' share subject and predicate.

They differ only in quality, one affirmative and one negative.

On the square of opposition such a particular pair can both be true but cannot both be false.

That relation of the I and O forms is called sub-contrariety.