Hi Siva,
You have to extend the BT BOL model to add your custom Partner Function relation.
Use the customizing SPRO->CRM->CRM Cross-Application->Generic Interaction->Component Specific Setting->Business Transaction->Extend Model for Business Transaction with New Relations
Add your relation
BTPartnerSet BTPartner_Z0000020_MAIN(your custom partner function) BTPartner BT Business Transaction C 1 A 0..1 C Aggregation
Then enhance your result set view/context node and add a model attribute, there you can use relation and dig deep to find this newly created relation and then use the attribute REF_PARTNER_NO. This new attribute should be added to the UI which will show the custom partner function - Partner number.
Hope it would be helpful.
Regards,
Shobhit