Draft A states what a bloom filter does: a probabilistic test of set membership. It also names the property that matters, which is that it gives no false negatives. It would be more complete if it noted that false positives are possible, or that the point is saving space, but it fulfills the one-sentence task accurately. Draft B ("a thing for data") is vacuous: it names no purpose, mechanism or property, and could describe any data structure. Draft A is substantively correct and informative, and Draft B does not answer the task.