Thanks, Tim, now I understand. Actually, in the Quiz Overall score the "Grade boundary" fields work on a dual system : percentage and/or total quiz score points.
Case #1.- If a quiz total grade is set to 100, of course, percentage and points are the same, and the Grade boundary fields will accept points as well as percentages (80 or 80%).
Case #2.- If a quiz total grade is set to 10 (the default value - and I wonder why the default value is 10 rather than 100), then percentage and points are the same, and the Grade boundary fields will accept points as well as percentages. The following sequence is accepted: 80%, 6, 40%. Strange! Upon saving the "6" value will be automatically changed to 60%.
Case #3.- If a quiz total grade is set to 20, then the following sequence is not accepted: 12, 8, 4 is not accepted and we get the error message "Feedback grade boundaries must be between 0% and 100%. The value you entered in boundary 1 is out of range."
The online help says "By specifying additional grade boundaries (as a percentage or as a number), the text shown can depend on the grade obtained."
This is not really clear, hence the confusion. Nowhere does it mention that when a grade boundary is entered as a number it cannot exceed the max grade score of the quiz.
Joseph