Hi V,
I had a similar case for one of my clients i will list the steps down and yes they are strictly utilizing ABAP related steps.
Assumption:
1. You want to give 10 % discount on header level
1. I will define a new discount condition(Header) say ZDIS and use a VOFM Requirement Routine for this
2. In the VOFM Routine, you need to check the item categories of all the line items utilizing the VBAP related structures. (ABAPer needs to check what all structures are available and have values). if more than 1 TAD item category is there, then your requirement is fulfilled.
Please note that VBAP related structures may not be filled during actual entry of line items..but will surely by available when you press SAVE and the requirement for the header discount condition should get executed at that time also.
This is a very broad idea about solving the problem, i am sure your ABAPer can refine it.