Check Allocation Matrix for an overview of the process.
The rule works as follows:
The system gets the amount going to P&L, ignoring everything (costs going to Process Order, depreciations (Credit (D90) and Debit) and fixed costs absorbed (E90) with exception of fixed costs of the period (Credit + Debit) and dividing Credit by Debit.
This is possible because destination type "STOCK_VC" is calculated by all cost posted on cost centers of node ECP from hierarchy ZCBS.