How do Parent Metrics work with Cascading Metrics?

There are two ways a parent metric can calculate the actuals and targets from it's children.  

Summation

This is the default roll-up type for cascading metrics. This roll-up type will add the actuals and targets from the child metrics together for a total in the parent metric.

Average

This roll-up type will calculate the average of the child metrics. The parent metric will show an average of actuals and target