in set of subactivities has a set of preffered...
when we use "split index".
Did not understand this point.
I think alfaromeo explained well, I followed the conversation with google translator.
If you have activity A, split into A1, A2, A3 and A4 (four lessons per week), then split index is 3 for A3.
Another way: you have 4 (sub)activities A1, A2, A3, A4, with ids: A1(id=104), A2 (105), A3(106), A4(107), and activity group id for all A1..4 is 104, then split index is 1 for A1, 2 for A2, 3 for A3, 4 for A4.
This is to constrain only some components. For instance, you need that from 4 hours per week, 2 to be in the first hours. Then add this constraint with split index 1 and 2 (or 3 and 4, or other combination).