I'm a teacher, and let's make this all simpler!

Assignment Group Weighting is used for big categories of grades. For example, you might calculate your student's final grades this way:

Homework Assignments 10%

Project A - 15%

Project B - 15%

Project C - 15%

Midterm Exam - 20%

Final Exam - 25%

Now, let's look at an individual project (e.g. Project :A). At this point, you cannot use Assignment Group Weighting. This is no sub-tier. At this point, you need to think in terms of points. So let's figure out points for Project A:

Outlining assignment - 10 points

First draft - 15 points

Final draft - 20 points

I hope that this explanation helps to clarify how Assignment Groups Weighting works.