What does "optimum range" mean?

[Dragonlord]

This leads me to another question on missiles...CSM missiles have the same damage no matter the range, once they hit then poof. But Plasma millies apparantly do less damage the further away you are, just like many beam weapons. But they are seekers?! How can this be? Once they hit, does the damage for 0-range apply? or the damage based on the distance between the launcher and the target at the time of shooting?

Also does multiplexing help missiles? Can it auto seek another target if its orginal target is destroyed? (in strategic combat).