Previously seen filter
Posts the viewer already saw are removed (impression history + a bloom-filter backup).
What it means for your posts
You don't get repeat impressions on the same viewer — a flop can't be re-served to the same audience; new content can.
A post removed by a filter is never scored at all for that viewer — no weight, however favorable, applies. This is why most “my reach died” experiences are filter stories, not weight stories. See the full filter pipeline and test a draft in the checker.