What is the tax source system code for benefits and overpayments?
SELECT T1.source_system_code  FROM CMI_Cross_References AS T1 JOIN Benefits_Overpayments AS T2 ON T1.cmi_cross_ref_id  =  T2.cmi_cross_ref_id
tax source system code refers to T1.source_system_code; for benefits and overpayments refers to joining CMI_Cross_References with Benefits_Overpayments on cmi_cross_ref_id;