Hi Kamalika ,
Go to transaction CRM_ORG_PROUVE. Enter the name of the Transaction. Copy the Rule Org. Unit and go transaction PFAC and enter the rule.
This is the excat FM that it behind the Org. Model Determination.
If you want to change the determination either you have to change the rule or you have to change the param in the Transaction Customizing.
~Kavindra