Suffer not the Alien to Live

The Chapter is expert at fighting a particular xeno-species. All models have the Preferred Enemy special rule, against either Tyranids, Eldar (including Dark Eldar), or Orks. This ability costs 1 point per model.
Every Space Marine non-vehicle model used must have this ability if any do. The choice of Preferred Enemy should be apparent from trophies and other embellishments on the models.