How IRR is calculated? what is the formula?

It is understood that Interest expense is calculated based on IRR rate instead of using user provided discount rate. In this case I am interested in IRR formula Oracle use for this calculation, it is very critical to business to provide the clear insight to Auditors about this calculation

Query used to find IRR rate:

select distinct(discount_rate) from pn_pmt_item_amrtzn_stream_v where lease_id = 12000

This query returns three rates, how to know which rate is been used for final calculation? what it the exact condition to get a single rate

Thanks,

Hari