1st Layer ODS - How to identify the Key Fields from extr. struct

Hi,

I am building some 1st Layer ODSses which should mirror the structure of the infosources 2LIS_11_VAITM & 2LIS_11_V_SSL.

However, I can't find a way to clearly identify which fields must be used as key field. It seems logical that I should include the Order Header and Position number but what about the FISCVARIANT. Is there a way to cleary identify these fields?

For example the 2LIS_11_V_SSL needs at least three fields becasue it is schedulte line level, but is this enough or will I lose (overwrite) information?