1
     / \
   10   20
   /  \
 200   300

If class 200, 300 and 20 are sending as much as they can, each class will receive 33%. So parent 10 don't get 100 (50%) like you should expect. However consider these settings :

Settings :
1 : rate = ceil = 200
10 : rate = 100, ceil = 200
20 : rate = 100, ceil = 200
200 : rate = 50, ceil = 200
300 : rate = 50, ceil = 200

If class 200, 300 and 20 are sending as much as they can, each class will receive it's rate. So class 20 gets 100 (50)%, class 200 and 300 gets each 50 (25%).
If only class 20 and 200 are sending, they each get 100 (50%).

Rule 3 (bis): sum (rate of childs) = parent rate
stef.coene@docum.org