The PO and DO scores of the Intercultural Development Inventory (IDI) are determined using separate validated, theoretically grounded proprietary formulas. Because the formulas are proprietary, they are not shared. However, the PO is an unweighted formula, while the DO is a weighted formula.
You can read more about the research behind these scores on page 4 of the following article:
Hammer, M.R. (2011). Additional cross-cultural validity testing of the Intercultural Development Inventory.
International Journal of Intercultural Relations. V35, 474-487.