The Header condition type works like this.
example :
condition type : ZTCT - HEADER CONDITION TYPE
If it is a header condition ,then you will be able to enter the value only at the header level . if you see at the item level ,this condition type will be on the display .
if your PO contains three line items , then the value you enter at the header level gets distributed among the item.( if i remember correct ,it can be distributed based on the quantity or Value.
Header value - 500
PO Item 1 QTY - 10
PO Item 2 QTY - 20
PO Item 3 QTY -20
Then the condition value you have entered at the header level gets distributed as
PO Item 1 condition value - 100
PO Item 2 condition value- 200
PO Item 3 condition value -200
sum of the condition's value - 500
So , if it is a header condition ,please enter the value only at the header level
Still if you doubts , reply