«ليس بعض»
Not all members satisfy the predicate.
«بعض ليس»
Some members do not satisfy the predicate.
Both are discussed in connection with particular negatives, but the commentary distinguishes their direct meaning from what follows by implication.
“Some students are not present” directly asserts particular negation. “It is not the case that every student is present” directly denies a universal affirmative; in the traditional square of opposition, under its standard assumptions, particular negation follows.