Sub Group within a Group

Hi,

I have a column describing contracts per process stage: Exported, WIP, paymenent processing and paid. So, I want to have a new categorisation, when selecting issued to calculate revenues from all 5 process stages and when selecting paid to calculate revenues from contracts in only payment processing and paid stages.

The problem here is that the paid group is also an issued one, so I need to somehow add this flexibility.

Thanks in advance