When SAS documentation such as at https://documentation.sas.com/doc/en/pgmsascdc/v_024/irpug/irpug_mirp_details03.htm
Has limited details like:
The OBJECTIVE=CREATEORDER option in the PROC MIRP statement is designed to support order generation. When multiple locations are ordering from one upstream location and the total order amount exceeds the available inventory at the upstream location, you have to decide how to allocate the available inventory among all orders. PROC MIRP uses marginal analysis to decide the best allocation based on service-level targets, demand volume, and current inventory status at the downstream locations.
one suspects much more than a simple "formula" is involved and the implementation of that analysis may need many pages of description.