1) In a grade category with aggregation method set to "Sum of grades", there is supposed to be a checkbox for setting extra credit for all grade items contained in the category. At least this is what the help file (aggregationcoefextrasum.html) says. There is a text field instead, and there is no clue for what value should be entered here.
2) Another inconvenience is that the extra credit setting is automatically set to 1 when switching from "Weighted mean of grades" to "Sum of grades" for a category, i.e. for all grade items with an item weight set. This is quite confusing IMO, I would expect 0 is the default value for extra credit.
Steps to reproduce #2:
Category C1; two grade items: A1, A2; two grades: A1 = 8/10, A2 = 2/10 for user ABC.
1. C1 aggregation set to Sum of grades.
2. Extra credit for A1 and A2 manually set to 0.
3. Grader report: ABC 10/20.
4. C1 aggregation set to Weighted mean.
5. Item weight set: A1 - 5, A2 - 10. (It has to be set to a non-zero value if the item is to be used in aggregation.)
6. Grader report: ABC 8/20.
7. C1 aggregation set to Sum of grades.
[8. Extra credit for A1 and A2 automatically set to 1!]
9. Grader report: ABC 0/20.
10. Extra credit for A1 and A2 manually set to 0.
11. Grader report: ABC 10/20.