1. The Definitions
* Addition (+): Child adds to parent.
* Subtraction (-): Child subtracts from parent.
* Ignore (~): Value exists but is not included in the parent's sum. Useful for "Memo" data.
2. The Report Card Example
* Addition (+): Math and Science grades add up to your total.
* Ignore (~): Your Student ID is on the page, but you don't want the system to add your ID number to your GPA!