Hi Zakiya,
Partner function(e.g Sold to Party, Employee Responsible, Contact Person etc) is a term used to describe Business Partner based on their function.PLease follow the link for more understanding.
https://help.sap.com/saphelp_crm700_ehp01/helpdata/en/46/3e6d80fddf2f91e10000000a1553f6/content.htm
So a business partner can be assigned to multiple partner function(1:n relation). Instead for a service order, u can get the partner details associated with the service order by using 'CRM_ORDER_READ' FM and from that u can get the BP number by passing the partner function number.
If this doesnt help you, please give more details about the place where u needed the partner function and business partner.
Moderated: Please avoid begging for points.
Message was edited by: Andrei Vishnevsky