Battle Strategies - Optimal Firing Range

[tbontob]

In the "Movement" section of the Strategies window, we are required to make a choice as to our primary and secondary movement.

Four of the choices include
-Optimum Firing Range
-Maximum Firing Range
-Short Firing Range
-Point Blank Firing Range

I assume, (correct me if I am wrong) that the following applies
-Maximum - weapons are fired at maximum range
-Short - weapons are fired at a medium range
-Point Blank - weapons are fired next to the target

My questions are:
1) What factors are used to determine for the optimal firing
2) What is the short or medium range?