Question: What are the register ids of electoral registries that have the cross reference source system code 'Electoral' or 'Tax'?
SQL: SELECT T1.electoral_register_id FROM Electoral_Register AS T1 JOIN CMI_Cross_References AS T2 ON T1.cmi_cross_ref_id  =  T2.cmi_cross_ref_id WHERE T2.source_system_code  =  'Electoral' OR T2.source_system_code  =  'Tax'