What does "optimum range" mean?

[Arak Koba]

Reading these Posts about multiplexing reafirms my belief that mixed fleets of large, medium and small craft is best. A large ship with massive firepower that doesn't have multiplexing could be overwhelmed by masses of smaller vessels at range with their defense bonuses. Plus all that firepower will be overkill against each small vessel individually, while the small vessels can mob their opponent.