How to get run-time delegated user id in Self service page

Hi

I have userA who has delegations of UserB. Record in Sysadmin is as below.

UserA Securing Attributes

Attribute	Application	Value
ICX_HR_PERSON_ID	Oracle iProcurement	2030
ICX_HR_PERSON_ID	Oracle iProcurement	2050
TO_PERSON_ID	Oracle iProcurement	2030

I have a query to retrieve registration number in mileage claim as one of DFF. My problem is when userA claim expense for userB , query retrieves userA's registration number. Could someone guide me how I can retreive userB's user_id in order to achive the solution.

Query

SELECT pvrf.registration_number

FROM pqp_vehicle_repository_f pvrf

    ,pqp_vehicle_allocations_f pvaf