CST - Reference Impedance & Power Conversion Efficiency in CST PIC Simulations

Hey all, I have had some issues in understanding the result tree in CST PIC simulations. The reference impedance shown in an electron beam device is showing what? Can any one elaborate and also guide what it should be for an X band vircator? Also is there any way to calculate the power efficiency of vircators.. i am using the example file available in CST Studio Suite provided by Dassault sys.