Hi Eli,
You are right that ZRVT is a Header condition and can only be entered @ Header level.
However, one thing to keep in mind is that the header condition entered @ Header will be passed on to the item. Here in your case, as your condition type - ZRVT has 1000 USD (consider you have 5 line items) then 1000 USD would be made applicable as part of the ZRVT condition type for all the items.
This is due to the reason that the calculation type is "Fixed Amount".
Please let me know for any clarifications.
regards
