GPA calculation formula details

We are seeing some odd results with GPA calculations for students who have unusual circumstances (incomplete at the end of the year, course dropped due to illness after the end of the year, etc).

The K-12 Help document (Create Grade Calculations & Rank) leaves out any explanation of the options that are at the heart of a GPA calculation.

Choose the parameters for the grade average formula:

• In the first box, select how the grades will be included
• In the second box, select what to multiply the grades by
• In the last box, select what to divide the grades by

We understand the options in the first box (we think), but the subtle differences in the options for the other two are unclear to us.

• blank vs Not applicable vs Credits attempted vs Credits attempting vs Credits earned
• blank vs Not applicable vs Credits attempted vs Credits attempting vs Credits earned vs Sum of Alt weights vs Sum of grades vs Sum of Weights

Does anyone have a reasonably clear description of what each option means?