Can we settle revenue related lower WBS to non-revenue related higher level WBS?

Hi experts

I require clarity regarding settlement of WBS elements:

1. Can we settle one WBS to other WBS at same level?
2. Can we settle revenue related lower level WBS element to higher level non-revenue related WBS? & vice-verse
3. What are the mistakes we generally make while configuring WBS settlements?

I will be very happy, if you clarify me these points which might look silly to you......

Thanks in advance my dear EXPERTS

Regards

Sanhitha