From Portals of Phereon Wiki

A special trait that can't be passed on. This reduces the ap cost for ranged attacks, essentially making the unit Agile if they can keep their distance from the enemy.

It's somewhat rare. You can see it on Eisvyl and Seliel, or Lava Spider and B Units. If you're a Mechanist, there's a special upgrade option which can grant it to certain robots.