passive crossover designer 5.1

There is a section of this program that asks for the listening / measuring distance, relative listening height, and verticle off axis angle in degrees. it also says all of these distances are "relative to summation point"

First, is the summation point the point at which i want the sound to appear to come from?

Second, what is verticle off axis angle?

Third, there is another section that asks for the driver's horizontal offset, verticle offset, and driver z offset. What does this mean?

This may not make sence unless you are familiar with the program, but any input is appreciated.